Mockupgen

Adds support for `black_white_point` option in templates which allows templates to specify how the screenshot colors should get shifted on the mockup.

In order to keep the size of the tool as small as possible, the tool now downloads mockup template files on the fly, from a different repository, [mockupgen-templates](https://github.com/rmenon1008/mockupgen-templates). This repository can be updated independently from the tool to allow the frequent addition of new templates. The option to specify a custom directory of templates is retained, and now a URL can be specified to, for example, a forked version of the default template repository.

First major release
Try installing the app with `pip install mockupgen`!

Adds new template options