io.ascii: make more flexible csv files

Data with missing values exported from Microsoft Exel as csv file can not be read with `astropy.io.ascii` in many cases. The reason is, that Microsoft Exel exports line-by-line, where each line only goes to the last entry. Thus, lines can be of different length, e.g. (in this example `col2` has a missing value)
```
col0, col1, col2
1,1,1
2,2
3,3,3
```
This requires an extra reader that overwrites `inconsistent_handler`.
It's probably done in a few lines of code.

The real questions is: Should this Reader be added to the list of Readers that are tried for `guess=True` (the default)?
On the one hand Exel tables with missing values are not uncommon, so this would help the "read everything" philosophy; on the other hand, it can hide serious errors in the input table.