adds permalink formatting options
Simplify permalink property

This mimics the `return 'none'` style as per @jnordberg's [comment][1] and
allows a global permalink style to be set in `config.json`.

It removes the *date* shorthand; I think its better to be explicit.

Related to #134.

  [1]: https://github.com/jnordberg/wintersmith/pull/147#discussion-diff-5628990
Add templating to page filenames (aka permalink templates)
Merge remote-tracking branch 'origin/feature-fntemplates'
Remove dependency graph