Allow multiple sourceHandlers in bundle

I wrote a Meteor-jshint package,

To run jshint on `.js` files is currently not allowed since there can't be 2 or more source handlers.

A source handler doesn't nessesarly change the code - Like in the jshint case - its just parsing and checking the code and reporting in on bad gramma.

I kept the warning about "two packages ..." - it only fires once pr. extension to keep down the verbosity