Allow different comment characters in one file
Unfortunately, I need to work with files containing both `#comments` and `!warnings` aside of useful numbers. Thus, it would be very convenient to load them (in pandas 0.17.1) using

`df = pd.read_csv(inputfile, comment='#!')`

however, this command now allows one comment-identifying character at most, so I need to parse the lines manually and save them into a temporary file first.

I guess it would involve rather a minor change to _search_replace_num_columns() in [parsers.py](https://github.com/pydata/pandas/blob/a292c13a7f831145f3daac9881813aeb7ff08138/pandas/io/parsers.py#L2329) and perhaps somewhere in [parser.pyx](https://github.com/pydata/pandas/blob/a292c13a7f831145f3daac9881813aeb7ff08138/pandas/parser.pyx). 

Before I write a patch, I would like to read your opinion.
