Add helm-dash support to use-package.

[canatella]

Add a :dash keyword to the use-package macro. This way, a user can ensure that a dash docset is installed and activated for a specific mode in its config:

(use-package ruby-mode :dash (ruby-mode "Ruby_2" "Ruby_on_Rails_5"))

That code will call helm-dash-ensure-docset-installed for the Ruby_2 and Ruby_on_Rails_5 docsets. It will also setup a hook to ruby-mode to add those to the buffer local docsets.

[jsmestad]

This is a really great feature to keep your dash docset definitions organized. 🎉