Add template option when generating packages or themes

It will allow other packages to provides their own templates and ease the process of creating plugins. 
For instance, I plan to provide a `Generate Plugin Package` command in the minimap to take care of the boilerplate, and I think other packages could use it, such `autocomplete-plus` that could generate a provider package using its own template.
Merge pull request #175 from abe33/patch-1

Add template option when generating packages or themes