Move pwa:create:screenshot to its own bundle

[tacman]

The screenshot generator is really cool. Besides dumping the yaml in the format that pwa.yaml expects, it doesn't appear to be specifically tied to the pwa bundle.

It seems like it might be a good candidate to move to a separate bundle, where the configuration could be focused on just screenshots. Some things that might be interesting to support:

• An event listener that generates screenshots from a database.
• screenshot:create --from-attributes could scan controllers for an attribute and generate pages (that didn't have route parameters).
• in addition to height and width, perhaps a screensize option, e.g. --screen=mobile|tablet|desktop

One advantage is that the screenshot bundle could be installed with the dependences as well, whereas now pwa:create:screenshot is only available if certain dependencies are installed, because simply installing the pwa-bundle doesn't mean the user wants the overhead of panther, etc.

Just brainstorming. It's a pretty awesome feature to have, thanks for including it!