"homepage" is not obvious; need an obvious way to generate HTML pages

Lineman is configured by default to generate my app's `index.html` page from `app/templates/homepage.us`. IMO this is non-intuitive behavior. It also makes it unclear if there is an opinionated way to add other HTML pages (Facebook requires my app to provide `channel.html` file, for example).

My suggestion is to add an `app/pages` directory. A "pages" task will copy anything from that directory into `generated`. The default `app/templates/homepage.us` would become `app/pages/index.us`, and be generated into `generated/index.html`.

I'm already building this for our app, and I'm happy to extract it.