Plugin can handle multiple generation languages

It's quite common to use Thrift to communicate software code in different languages. For this we can use the same Thrift definitions and generate the code for all the languages that we need. 

The proposed change edits the _generator_  property to accept a list of generators, the system will create a new folder for each of the languages passed in the _target/generated-sources_ folder. For back compatibility purposes I have not changed the name of the _generator_ parameter, either its default value (java:hashcode).