Add foodcritic syntax checker for Chef files

Requires a ftdetect plugin to set filetype to chef or ruby.chef, e.g. vim-chef (https://github.com/dougireton/vim-chef)
Minor help re-wording.
Add filetype and subchecker args to foodcritic makeprg
Merge pull request #821 from dougireton/chef_foodcritic

Add foodcritic syntax checker for Chef files
Added Chef to the list of supported languages.