Support template customization

[MarkRobertJohnson]

I wanted the ability to customize and override the default templates without modifying the source code for docgen. This pull request adds the ability for the user to specify a templates folder. The scaffold command will now by default copy the default set of docgen templates to the scaffolded folder. The "run" command will by default look for the "templates" folder in the input folder, if it exists, then those templates are used, else the default set of templates is used.

The "scaffold" command will also copy the "require" folder to the scaffolded folder. This allows the user to customize some of the content in that folder (CSS etc.)