Per-collection URL base path

A collection's URL prefix is its name: entries live at /<collection>/<slug>. cloudhal.com wanted its blog at /blog while the starter's collection is posts, so it renamed the collection (the store's 301s from /posts/… were a nice touch). collectionIndex only affects menu links.

Proposal: an optional base path per collection in the site settings, with redirects when it changes. At least, say this in the starter README.