Create R.sublime-completions

Hi,
I was going to create my own repo of completions, then I found your package (I really like your `Enhanced-R.tmLanguage`, BTW) so I figured I'd just add it to yours, if you don't mind.

I generated the command list by running `ls(pattern="*", baseenv())` then weeding out non-alphanumeric-beginning commands (`:::`, `&.octmode`, etc.) and single-character commands (`T`, `q`, etc.). Finally, I merged the list with http://dl.dropboxusercontent.com/u/2223411/R.sublime-completions (unknown author).

Please let me know if you need anything else.

Matt