Reactivate errorsOnly and use gulp-util in plugin.

Reactivate errorsOnly setting lost as a result of #15 (35e257cbf5741cfc729daf9c7ea21819a076be44).
Migrate code from using `Error` objects to `PluginError` objects in __gulp-util__. This allows __gulp__ to hide meaningless stack traces.
Migrate code to use `log` in __gulp-util__ rather than `console.log`. This allows for a consistent format to plugin console output.