add --pretty option for human readable timestamps

If this option is supplied, with or without any value, then values in the time column will be formatted using ruby's default to_s method on Time, eg:

```text
$ influxdb-cli db --pretty
...
✔ ready
[1] pry(main)> select isdir from /.*/ limit 2
+---------------------------+-------------------+-------------------+
|                              foobar                               |
+---------------------------+-------------------+-------------------+
| time                      | sequence_number   | isdir             |
+---------------------------+-------------------+-------------------+
| 2014-03-25 15:20:35 +1030 | 106770001         | false             |
| 2014-03-25 14:34:21 +1030 | 106760001         | false             |
| 2014-03-20 10:05:28 +1030 | 36650001          | false             |
| 2014-03-20 10:03:49 +1030 | 36630001          | false             |
+---------------------------+-------------------+-------------------+
4 results found for foobar
```