Make Liquibase migrations file an optional CLI argument

Sometimes it's desirable to maintain/package your database migrations outside of your application JAR (e.g. when the migrations file is massive).

Lets add an optional argument to all Liquibase Commands that permits specifying the path to the migrations file in the local filesystem.

Omitting this option retains the default behaviour.

Note: I'm not entirely sure how best to test Command arguments, so there are no tests. I used `dropwizard-example` to check that it roughly works as expected, but that doesn't really qualify as testing.