new parallel option

When using a grunt task like grunt-contrib-watch, the call to done() stops terminates grunt, and therefore the watch task.

A solution would be to not execute the rewrites using async.parallel, but in an iteartive way, despite it being less optimized.

The "parallel" option can be used to do just that.

Since the best way is still using async.parallel, the default value of "parallel" is 'true'.