Add pdftables-render --ascii for rendering table (and fix docopt string)

Example output:

```
$ pdftables-render --ascii s/unica-01-09-13-page-3.pdf 
Rendered svgs/unica-01-09-13-page-3.pdf_00.svg pngs/unica-01-09-13-page-3.pdf_00.png
     10 columns, 25 rows
                    0           1                  2      3                     4                                   5      6            7                    8      9
    ------------------------------------------------------------------------------------------------------------------------------------------------------------------
  0 | Bi-weeklyperiod| 2012/2013S|Vão Paulo2013/2014|ar.(%)|SUGARSouth-C2012/2013|CANE (tons)entral region V2013/2014|ar.(%)|2012/2013Oth|ers states2013/2014V|ar.(%)|
  1 |           16/04|  2,130,641|         6,091,407|  186%|            4,736,769|                          9,520,749|  101%|   2,606,128|           3,429,342|   32%|
  2 |           01/05|  6,753,397|        26,345,485|  290%|           14,133,076|                         41,478,714|  193%|   7,379,679|          15,133,229|  105%|
  3 |           16/05| 18,078,715|        52,476,576|  190%|           35,014,193|                         81,663,266|  133%|  16,935,478|          29,186,690|   72%|
  4 |           01/06| 40,950,372|        73,770,632|   80%|           70,798,058|                        117,013,110|   65%|  29,847,686|          43,242,478|   45%|
  5 |           16/06| 55,094,486|        94,908,421|   72%|           96,674,881|                        152,472,389|   58%|  41,580,395|          57,563,968|   38%|
  6 |           01/07| 73,838,517|       112,806,258|   53%|          128,377,825|                        181,712,259|   42%|  54,539,308|          68,906,001|   26%|
  7 |           16/07|100,037,254|       138,787,355|   39%|          170,572,914|                        224,470,485|   32%|  70,535,660|          85,683,130|   21%|
  8 |           01/08|128,355,725|       165,206,366|   29%|          216,850,039|                        268,662,457|   24%|  88,494,314|         103,456,091|   17%|
  9 |           16/08|155,949,298|       193,143,006|   24%|          261,096,423|                        314,915,251|   21%| 105,147,125|         121,772,245|   16%|
 10 |           01/09|185,033,425|       222,604,594|   20%|          307,615,408|                        363,453,465|   18%| 122,581,983|         140,848,871|   15%|
 11 |           16/09|           |                  |      |                     |                                   |      |            |                    |      |
 12 |           01/10|           |                  |      |                     |                                   |      |            |                    |      |
 13 |           16/10|           |                  |      |                     |                                   |      |            |                    |      |
 14 |           01/11|           |                  |      |                     |                                   |      |            |                    |      |
 15 |           16/11|           |                  |      |                     |                                   |      |            |                    |      |
 16 |           01/12|           |                  |      |                     |                                   |      |            |                    |      |
 17 |           16/12|           |                  |      |                     |                                   |      |            |                    |      |
 18 |           01/01|           |                  |      |                     |                                   |      |            |                    |      |
 19 |           16/01|           |                  |      |                     |                                   |      |            |                    |      |
 20 |           01/02|           |                  |      |                     |                                   |      |            |                    |      |
 21 |           16/02|           |                  |      |                     |                                   |      |            |                    |      |
 22 |           01/03|           |                  |      |                     |                                   |      |            |                    |      |
 23 |           16/03|           |                  |      |                     |                                   |      |            |                    |      |
 24 |           01/04|           |                  |      |                     |                                   |      |            |                    |      |
    ------------------------------------------------------------------------------------------------------------------------------------------------------------------
```