Expand PluginInterface support

I just wrote a plugin for [scalajs](http://www.scala-js.org/) (very rough but works), using the plugin interface and two things would be super nice to support.

- Extra schema, like in the relayCompilerBin you add directives, it would be nice for a plugin to be able to add custom directives.
- The getFileFilter function looks for the string graphql, we should be able to override that.

Otherwise, it went well.

Start by reading the PluginInterface and the relayCompilerBin handling mentioned in the issue, then trace how getFileFilter identifies GraphQL files. The work is complete when plugins can add custom directives and override the file-filter string, with coverage for both extension points.

Written by the indexing model from the issue text.