ENH: Add option to Copy Theme files to external folder for testing

[Timo-Breumelhof]

Is your feature request related to a problem?

Please describe.
We generally develop the Module outside the DNN folder.
For Themes this is not the best solution.

Describe the solution you'd like

Add an option to copy the Theme files (on gulp build) to one or more "external" folders to make developing Themes easier

[Timo-Breumelhof]

@johnhenley ok one for my own list then.. ;-)