Pass an opts hash up to Mongoriver::Tailer's constructor

...and roll the oplog argument into the opts hash in cases where
people want to override it.

This is an API break, but only a minor one (but we should be sure to
bump mosql's dependency on mongoriver).
Allow specifying a query comment for Mongoriver::Tailer objects

Mongo query comments effectively allow to you tag queries with
arbitrary extra structured information that shows up in both the
current op list and profile logs. We use this to correlate queries
with other information like requests and users.