Moved embedded files into separate file - added library support

Big update! This moves the generated files (app.js, index.html, etc) into an external file, so users can change that file's contents without touching up the plugin code.

The main motivation for this was enabling a library of common FramerJS functions I use. When configured, sketch-framer now downloads a remote library and includes it in index.html.

(We could also use this so stop curl'ing Framer remotely and just including it in the include file?)