Rewrite, more or less

Hello,

I came across vim-rspec-focus while looking for a plugin that would do the add/remove/toggle focus stuff I'd had in my vimrc for a while. I forked your plugin and ended up rewriting most of it. I kept the original API for AddFocusTag and RemoveAllFocusTags, but changed the implementation and added a few other niceties like describe-, context-, and it-aware functions and the ToggleFocusTag() function.

Please let me know if you would considering merging in this pull request, or if you would prefer that I just split off the repo entirely.

Cheers,

Mark