Add support for MariaDB

[jsongerber]

This PR allows this package to work with MariaDB.
In MariaDB, --set-gtid-purged on mysqldump does not exist and throws an error.

Here is the changes I made:

• Added REMOTE_DATABASE_TYPE config variable, with the default value to mysql, so there is no breaking change
• If REMOTE_DATABASE_TYPE is set to mysql, --set-gtid-purged=OFF is used, otherwise it is not
• If REMOTE_DATABASE_TYPE is set to mysql, mysqldump is used, otherwise mariadb-dump is used

There's some observation/question where you maybe can help:

• The existing config variable REMOTE_DATABASE_MYSQL_HOSTNAME use mysql in its name, which could be confusing now that MariaDB is supported, not a big deal and changing it would be a breaking change. A solution could be to have a new REMOTE_DATABASE_HOSTNAME (without mysql) variable, and throw a deprecation notice when the old one is used.
• REMOTE_DATABASE_TYPE is not strict, only mysql uses --set-gtid-purged=OFF, any other string is not using it. So using mariadb would have the same effect as foobar.
• No error is shown if the dump fails, but that's probably out of this scope.
• mysqldump is still available with MariaDB, but deprecated, mariadb-dump is replacing it.
• I didn't add tests.

Closes #30