Add the possibility to provide the content to process

I've just written a Gulp plugin ([gulp-htmlprocessor](https://github.com/Nesk/gulp-htmlprocessor)) based on this project and providing the same features as [grunt-processhtml](https://github.com/dciccale/grunt-processhtml).

However, to achieve this, I had to slightly edit the standalone library. In the Gulp way, plugins rarely take file paths as inputs, but their contents. So I just added a `processContent()` method which requires the file contents and its path.