Add org.vim.rst-file to to LSItemContentTypes

I don't know if this is right, but it gets quicklook to use the qlrest
plugin when macvim is installed.

toland/qlmarkdown#3 looks like a related fix for a macvim markdown type
conflict.
Merge pull request #1 from groner/patch-1

Add org.vim.rst-file to to LSItemContentTypes