Add Socket option in Custom MySQL Configuration

[anmolrajpal]

Though there are abundant configuration options in the current version, however, I want to use a custom socket for my configuration. As I am using MAMP, it runs MySQL with a custom configuration with a custom socket => /Applications/MAMP/tmp/mysql/mysql.sock
Other Server Side Swift Frameworks such as Perfect Swift provides this 'Socket' option.
I hope this option will be added in Vapor too.

[tanner0101]

Related to vapor/postgres-kit#170