Helper methods for tsvector_column (auto)rebuilding

Hello!
Please see updated [README](https://github.com/marshall-lee/pg_search/blob/tsvector_autoupdate/README.md) for details.
Summary: It makes possible to avoid creating database triggers and, in some cases, avoid using `:sql` schema format. For example, in my project i use only `pg_search_scope` feature and none of pg_search features that require writing raw sql migrations, so i can use nice and clean `schema.rb` instead of raw sql. Now i do tsvector rebuilding in `after_save` callback which is the right way to do such things rather than using postgresql triggers.

If you ready to take this, i can go further and implement support for `dmetaphone` in similar way. Please let me know what i might fix or refactor.
I noticed that you only add the rebuilding functionality for Hash-based options, not Proc-based options. I'm assuming this is on purpose because I can understand how you can't statically know the correct configuration when a Proc is involved.

So I'm OK with it, but I think it's important that this behavior is documented. Ideally, the `:autorebuild` feature would raise an error at runtime if it was set to true from a Proc, unless we can think of some way to make it make sense there.