Add support for multiple databases

Simon provides support for using multiple databases. By adding `prefix` and `alias` parameters to `init_app()`, Flask-Simon is able to support multiple databases as well.

`prefix` can be used to point Flask-Simon at different configuration settings. `alias` can be used to control the alias that Simon will associate with the database. If no value is provided, Simon will use the default behavior of using the database's name.

`init_app()` now contains a nested method called `prefixed()` that can be used internally to get the full name of a configuration key.

Closes #2
Document prefix and alias

The documentation being added is for arguments added in a4c8116.

References #2
Typo