Frontmatter defaults

This is a WIP towards general defaults for YAML frontmatter variables, as @benbalter suggested in [#969](frontmatter_defaults). The defaults are specified in `_config.yml` and can be restricted by type (post, page or draft) and path.

## TODO
- [x] basic implementation
- [ ] more robust cucumber features
- [ ] inline code documentation
- [ ] documentation for jekyll.rb

@mattr-, @parkr what do you think?