Fix migrationsPath option handling so that projects using the plugin can actually override it

The assumed relative path in default options was wrong, and setting the migratorOptions.path value as a relative path from the plugin's tasks directory is probably a bad idea, as it prevents anyone from completely overriding this path with a custom, absolute path value.

I've made modifications to fix that and checked that tests were still passing. Hope that's all right!

Fixes #2