Added support for specifying colors in parallel_plot()
Sometimes the random colors chosen in parallel_plot are very unfortunate, so I thought it would be nice to provide an option to pass a list of colors in.

Looks good to me. Can you please add a test case so I can merge it in? Thanks!
 Sure thing. How does this look?

I have one more related pull request I was planning on submitting after this which enables the x-axis to be rescaled when numeric column values are used (e.g. "# hours"). Is there anywhere I should go first to suggest the change, or is it okay to just submit another pull request?

Thanks!
 If you already coded it up, feel free to make another PR. What's the current behavior that's not right?
 Cherry-picked. Thanks very much!
 Great! Currently parallel plots remaps the x values to 0, 1, 2, 3... so the plot is also scaled linearly. It wasn't too hard to change it so that if all of the columns are numeric it uses those as the xticks. This way when you pass it something with columns that are not evenly-spaced the plot is scaled correctly.

I'll go ahead and submit a pull request.
 Oh I see, that makes sense. Maybe make that an option instead of doing it automatically? Like a 'use_columns' parameter analogous to 'use_index'.
Also maybe another optional parameter to pass in an external set of xticks so if you already have meaningful column names, you don't have to mangle them to get the spacing right?
