please allow excluding schemas from check_bloat (or generally)

[justinpryzby]

We have a schema which has many small, transient tables. They're never updated, and if they have poor storage use (due to alignment), it's of no concern (because they're relatively small) and there's nothing we could do about it anyway. Currently nagios can time out and then sends spurious notifications about "OK - check_bloat" when it rechecks.

I saw that it's possible to filter tables on the client side, but that doesn't help to avoid the query timing out.

Ideally this would be a general facility for excluding schemas/tables (and not just DBs). See also #176 and #49 regarding filtering queries.