Autodetect nav manu for each page, enhance with page's frontmatter

[afred]

Description

To make menu management as easy as possible, we want all the management to occur in one place, which we're putting in _data/nav/.

Each file in here will be a YAML file containing nav items for display that are relevant for a given page.

We want content editors to be able to just put in the path or URL of a given page at the desired location within the nav tree in one of those files under _data/nav/ and just have it work.

This means we'll need a tool to search though the avalable navs, and figure out which one applies for a given page -- AND -- apply the page's frontmatter as default values to the nav items (i.e. default to use page title for the nav item's title).

Done when

• page['nav'] (or page.nav depending on implementation) contains the correct nav tree in the context of liquid templates.
• The nav item representing a given page had default values that come from page's frontmatter