Compile external thrift files.

We've started using the sbt-scrooge plugin to add scala source generation to some of our projects that use thrift + scrooge. Thanks for bringing the plugin back to life! We need to share some thrift files between projects and, rather than use sub-modules, we'd like to publish thrift files in jar files and then include them in the projects. The individual projects should take responsibility for compiling the thrift files.

JAR files containing thrift files to compile should be added to the list of library dependencies, but in the "thrift" configuration. The plugin will unpack the thrift files from the JARs to an external source directory and then compile them along with regular project thrift files.

Should this pull request (and any future ones) be done against bancek/sbt-scrooge or koofr/sbt-scrooge?