Preliminary MySQL support. Barely tested.

Requirements:
MySQL/MariaDB need to be configured with this settings:

  innodb_file_format = Barracuda
  innodb_file_per_table = On
  innodb_large_prefix = On

to support key prefix length up to 3042 bytes.

MySQL/MariaDB DSN will need this parameter:

  parseTime=true

as per [1]

The migration system itself mostly inspired by Rails (ActiveRecord),
but it still rough at the edges. Could use some inputs.

Next Todo: more testing!

[1]  https://github.com/go-sql-driver/mysql#parsetime