migrator constructor doesnt allow for schema prefix

[DanielFGray]

rather than assuming on the default public postgres schema, the constructor should allow users to define a different schema for the migrations table to live in, separate from the schema that the migrator populates tables in

[mmkal]

You can pass an array instead. e.g. migrationsTableName: ['something_other_than_public', 'migrations']

an improvement to the docs would be greatly appreciated!