v0.5.0 - Tracking Pull Request

This is a PR to track changes happening on the v0.5.0 branch.

Please feel free to test this out, submit bug issues and provide feedback on the API.

***
TODO:

- [x] collections
- [x] sorting on collections
- [x] index pages for collections
- [x] add `mixins` option
- [x] make handlebars a first class template engine
- [x] use [plasma](https://github.com/jonschlinkert/plasma) for loading data, helpers, mixins and plugins
- [x] use `defaults.yml` for assemble defaults. (see: https://github.com/assemble/assemble/issues/258)
- [ ] `.assemblerc.yml` (see: https://github.com/assemble/assemble/issues/505)
- [ ] API documentation
- [ ] CLI
- [x] better support for building pages from files or JSON
- [x] better support for building components
- [x] improved logging
- [x] granular access to context, at any time during the build
- [x] plugins can define the stages in which to run
- [ ] helpers: option to disable built-in helpers 
- [ ] helpers: API consistent with plugins