Allow passing other arguments to interpolation functions
Closes #10378

can you add some tests?
 Pls update docstring in `generic.py`.
 Okay. I added tests and updated the docstring. In looking more closely I'm actually not sure what other interpolating methods should be passed the kwargs. Potentially `piecewise_polynomial_interpolate` but I don't really understand it (#10365). I think `interp1d` is getting all of the relevant variables.
 Anything else to get this merged? And wow, travis-ci is SLOW at running these tests.
 see here: http://pandas.pydata.org/pandas-docs/stable/contributing.html

pls squash.

and travis seems quite chipper lately. We have a quite complicated setup, so takes a bit.
 Ok, I'll update the docs and then squash. I usually wait until code is :+1: before squashing to make reviewing easier.
 Squashed and docs.
 Anything else remaining to get this merged?
 thanks @cancan101 
