Support a gulp-babel drop-in compatible mode

[Marcin Szczepanski]

Currently gulp-parable will read the source file itself to allow for caching, however there's no way to pipe output from another gulp task into gulp-parable.

My use case: I am playing with Typescript, but still want to use Babel to transform from ES6 -> AMD modules (because I have specific naming stuff I already have setup in Babel). I can't pipe the JS output from TS to gulp-parable because it'll try and read the original source file. I don't want to also have to depend on gulp-babel just to do this.

This would require support in parable itself to be provided the source for a single file as opposed to reading from a file (something like processor.transpile would need to support source being source as opposed to a filename?)