create custom extension for semantic-sql

[wdduncan]

Related to #21

If we want to "trick-out" semantic-sql with specialized functionality, it may be worth creating a custom sqlite extension for semantic-sql. I am not sure how hard this would be.

Another option (perhaps) is to use peewee to make use of the extensions. Here is some info about peewee extensions:

http://docs.peewee-orm.com/en/latest/peewee/sqlite_ext.html#sqlite-ext
http://docs.peewee-orm.com/en/latest/peewee/playhouse.html