Use filetype to determine whitespace stripping.

The main reason is to allow each filetype to specify which patterns
match. For example ruby files can be more than just '*.rb' (Gemfile,
*.rake, etc). 

This change also allows the user to override 'g:spacejam_filetypes' with
a different list of filetypes. This makes the list of filetypes
configurable.

I have also used the
[vimrunner](https://github.com/AndrewRadev/vimrunner) gem to test the
plugin.  

There is still an error with the spec for user overrides, but I'd like
to get some input on whether this direction will be accepted before
spending too much time trying to figure it out.