add r_in_tagged_strings

[TyberiusPrime]

I often use rpy2 to use R from python - this highlights all strings tagged with #R in their contents.

Thanks for building this btw., it was really nice to get started hacking with this!. Very shallow 'try it out, here's how to do it' curve!

Now if I only could get neovim to use different themes for different injected languages :).