Add support for Postgres's plainto_tsquery
I've made a slightly more compact patch for the actual implementation, plus some improvements to the docs and added a couple tests:

fa8fc729f37b1ab4da420fb51f8bbc90d4074fb7
peewee's `TSVectorField` helpfully includes a `match` method to add a `tsquery` to a query using `to_tsquery`. However, `to_tsquery` is one of two ways you can convert a string to a `tsquery`, per the [Postgres docs](https://www.postgresql.org/docs/9.1/textsearch-controls.html):

> PostgreSQL provides the functions to_tsquery and plainto_tsquery for converting a query to the tsquery data type. to_tsquery offers access to more features than plainto_tsquery, but is less forgiving about its input.

This pull request adds a `use_plain` parameter to `TSVectorField.match`, thus letting the programmer choose whether to use `to_tsquery` or `plainto_tsquery`. That's especially helpful when you're writing an application with casual user input for a search query.