Adds support for MySQL SSL connection kwargs
Addresses https://github.com/coleifer/peewee/issues/1742

To configure SSL connections with MySQL,`MySQLdb.connect` expects a dictionary of SSL params. Currently, `playhouse.db_url`  logic provides no way of parameterizing an SSL connection via the URI. This pull request apes `SQLAlchemy` by letting users specify nested ssl params in the query string with `ssl_*`. 

There should be no backwards compatibility issues, because `ssl_` prefixed query params would've previously had no effect.