Add support for MySQL unix_socket connection option

[coryvirok]

Connection strings that specified mysql:///db?unix_socket=... option were not passing the unix_socket option to the underlying aiomysql connection. This meant that it was not possible to connect via UNIX domain socket unless the connection option was explicitly given to the MySQLBackend() constructor.

This change adds the unix_socket connection option to the explicit list of connection options recognized and extracted from the connection string.

[coryvirok]

I tested this by running a local MySQL server and adding a ?unix_socket= connection option to the TEST_DATABASE_URLS.

E.g. TEST_DATABASE_URLS=mysql://root@/test?unix_socket=/tmp/mysqld/mysqld.sock

[coryvirok]

@tomchristie any chance you or another maintainer can review this? Thanks

[afgarcia86]

Bump I could really use this change as well

[ryanrasti]

What's the status here? I think URL has support for the unix_sock parameter which makes this change simpler. I have a PR of my own: #503

[zanieb]

Superseded by https://github.com/encode/databases/pull/551