Add ArcanistConduitLinter, a linter that delegates through Conduit

Summary:
Julien built a really cool static analysis database of our codebase. One
capability is that it can suggest typehints that are not in the code. The
analysis to do this is very expensive, so it can't reasonably be run locally.
But it can remain indexed on a server.

The idea here is to provide a familiar interface to it through arc lint, via a
generic Conduit service call.

In our lint engine, this will probably be gated on --advice for performance.
This will introduce a slight awkwardness in that running with --advice can add
new non-advice lint if the server chooses, but this isn't likely to cause a
practical problem.

Test Plan:
Construct a fake Conduit lint endpoint, attach this linter to it, and see bogus
lint
appear with --advice.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D1462