Provide a Conduit method to get inline comments

Summary:
See D2855 for usage.

This has a drawback that inlines without a comment (synthetic comments) are not attached anywhere.
I don't like adding more and more methods so I've chosen this solution.
Plus comments and inline comments are often useful together.

Test Plan: Called the method on a revision with inline comments.

Reviewers: epriestley

Reviewed By: epriestley

CC: vii, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2844