Added code for determining the current project root directory

- The Mailgen app now detects the root directory of the active node project before executing commands. Refactored much of the Mailgen bootstrap code while implementing this feature

- Rewrote index.js to return a "MailGen" constructor, which is used to create the main application object (so far just containing basic settings). Settings like the source and build directories have moved here from the Scaffold submodule

- Rewrote the Scaffold submodule to use the application object for settings like source and build directory. Also removed the requirement for Scaffold to run in the project root (no longer required as absolute paths to the root will always be used)
Moved Scaffold's dependencies to the top of the file
Added a Build submodule to handle building emails

- The Build submodule can build the HTML of emails from their source. It handles including of utilities from the library, inlining of CSS, and minification of HTML
Standardised property names in the main app object