View source of single panel should call only its own preprocessor.

At the moment when we call something like bin.x.css and its content is of a preprocessed language, it will call all preprocessors connected to that bin.
With this fix it will call only the single selected processor.
Changed the logic to pass only one preprocessor.

The bin settings.processor object now contains only the select panel/processor before going through the applyProcessor function.
Merge pull request #1737 from jsbin/fix/single-panel-processed-source

View source of single panel should call only its own preprocessor.