Add support for comments

[furious-luke]

This adds initial support for Postgres comments. At the moment it's only working for comments on functions, but it won't be hard to extend to other objects (just need to update comments.sql).

[djrobstep]

Nice! With some tests that run this code and pass it I'm happy to merge it in at that point.

We can extend the comments.sql query later to support comments on the various other entities (or include them in this PR if you want, up to you).