Add support for unpacked extensions and user scripts

Addresses #5 (closed)

Electron cannot currently load regular Chrome extensions or .crx files, but it does support unpacked extensions. There are Chrome extensions out there that will help you unpack installed extensions.

This merge request adds support for loading unpacked extensions from the user's APPDATA/Twitch/Extensions directory. Simply unpack an extension (like BetterTTV), place the folder in APPDATA/Twitch/Extensions, and it will automatically be loaded on app startup.

Also adds support for user scripts. The user can save scripts to files in their APPDATA/Twitch/Scripts directory and it will be automatically loaded on startup.

Also added a node-based gitignore file (to help ignore node_modules and other stuff) and updated the README.