refactoring to new plugin format

[alexkaessner]

Would be nice to have this plugin as the new sketch plugin format so we can see it in the plugin preference pane and also check the version number for updates. This also allows us to open the plugin documentation inside this pref pane.

Btw, a small introduction window directly inside sketch would be even more useful 😄

[auxdesigner]

@alexkaessner Thanks for the feedback! I'll take this into consideration as I do want to keep improving it. Thanks again!