Added the `g:vroom_rspec_format` flag

Ok so I added the `g:vroom_rspec_format` flag. I made it quite differently from the previous suggestion I gave, cause I found it would have conflicted with the `g:vroom_use_colors` flag. Adding a generic options flag would have forced a rework of the colors options, so I prefered to add a format flag.
It is preinitialized with the value of `progress` (cause it works both with rspec 1.x and 2.x, while documentation is called `specdoc` on some old versions and `documentation` on the 2.x). You can override this behaviour simply adding `let g:vroom_rspec_format='documentation'` in the vimrc.
Let me know if this works for you, I've tested it on my mac and it works perfectly. If you need anything, just ask :)