add some relevant r snippets

using a regular expression snippet is not ideal here as you do not really need it - it will not be listed for partial completions for example. And you do not really need it here - just pull out the common python code into the global above. 

Also, you replicate the complete method somewhat, use it like this:
https://github.com/honza/vim-snippets/blob/master/UltiSnips/proto.snippets#L35