configurable permalinks support

A sample config.yaml for a site might contain:

```yaml
permalinks:
  post: /:year/:month/:title/
```

Then, any article in the `post` section, will have the canonical URL
formed via the permalink specification given.

Signed-off-by: Noah Campbell <noahcampbell@gmail.com>
Documentation updates, mostly for my bits

* extras/permalinks.md added, weighted to third in the extras menu
* examples added to layout/go-templates.md, using `.Site.Params`
* mention `.Site.Params` in layout/variables.md
* update meta/release-notes.md to mention `first` and the permalinks
* update overview/configuration.md to use reserved-for-documentation
  domains and with another example, nudging towards permalinks and site
  parameters, with three different data-types shown for the latter

Signed-off-by: Noah Campbell <noahcampbell@gmail.com>