Option to change classnames across files in project

[XXXVII]

This feature might be very useful in creating widgets with unique classnames. For example, use case is to add prefixes to classnames across the project.

[cazzer]

Cool idea, this could be added to the library module pretty easily, the issue is in exposing that to the run call.

Currently, the ignores object is the only way to manipulate the libraries. Since we want to expose more of the libraries functionality ignores should be nested into a bigger library configuration object which could take a prefix property too.

To avoid breaking the current API I'll work on adding a parameter for run which takes more library options, but if you have any suggestions on how to structure the run API for the next major version let me know.