Allow asynchronous linting providers

[peterflynn]

Some existing Brackets extensions do linting via a web service, so they can't use the new generic linting API because it requires a synchronous result. The API should allow providers to return a Promise.

These extensions include:

• https://github.com/jeffslofish/brackets-PHPCS-client
• https://github.com/davidderaedt/appcache-gen

This would also allow for linters that invoke Node-side modules or even external tools (e.g. ruby -wc).

[njx]

Marking move to backlog since this is a feature, but pinging @ingorichter since he's thinking about this stuff already.

[njx]

Added an epic for Linting (https://trello.com/c/HbygrWHv) and linked this from that card. Closing.

[fdecampredon]

I would like to work on this issue : see the google group thread https://groups.google.com/forum/#!topic/brackets-dev/h-KE1Cc0cdY

[peterflynn]

Check out my notes in #6151 -- that highlights some the issues to be aware of. Also note that @busykai from that pull request may already be working on implementing this. If so, @busykai can you post an update to the forum thread @fdecampredon linked above? That way you guys can coordinate / collaborate as needed :-)

[busykai]

Thanks, @peterflynn will post to the group thread.

[peterflynn]

This is now in master -- Sprint 38