Add optional colspecs parameter to fwf

[susanxiong]

Closes #63

needs to be tested

[jayckaiser]

@susanxiong We're not ignoring your PR. There's a broader discussion to be had about how to improve user-flexibility when adding keyword arguments to file sources, but without having to update Earthmover with every possible argument.

[susanxiong]

@susanxiong We're not ignoring your PR. There's a broader discussion to be had about how to improve user-flexibility when adding keyword arguments to file sources, but without having to update Earthmover with every possible argument.

Yeah that makes sense! I haven't marked as ready yet because I also don't know if it's even worth it vs just preprocessing to a csv.