Adding optional classifier support to artifact ids

The plugin supports maven style artifacts identified by `group_id`, `name`, `extension` and `version` fields. It would be useful to support the optional [`classifier`](http://maven.apache.org/pom.html#Dependencies) field as well. This could be used, for example, to download sources or javadoc versions of an artifact.

Very new to coffeescript so hope this is ok! Thanks for contributing the plugin.