Cleanup

* Show error on streams, since they're not actually supported
* Remove unused function names from anonymous functions
* Use callback functions directly instead of local names
* Move options || {} directly into the _extend call
* Use PluginError directly
* Add missing semicolon
* Spacing
Bug fixes:

* Push null file instead of null literal when file.isNull()
* Re-use file and push it so it can also be processed
* Return next() so the next gulp plugin gets run
* Upgrade to through2

Conflicts:
	index.js