0.6.0: simplified, added front matter support (breaking)

* support YAML front matter, like Jekyll
* use a custom templating function loosely based on Lodash's _.template
* removed the Lodash dependency
* templates always run in strict mode; implicit globals and `with` are no longer supported
* removed support for `extend`-style inclusion; just include header and footer partials instead
* better default delimiters
* "active link" utils now prefer `aria-current`