Multi pages

Arg!!! lost my previous comment....

This PR includes the collections/pages PR since I didn't do that on a different branch.
This PR also includes the ability to define pages in the assemble options as an array of configuration. This would let you define them in a .json or .yml file for inclusion too (the task does not automatically read them in yet).

I changed `metadata` to `data` so it's more consistent with how pages data context is handled.
There's an example (`inline_pages`) in the Gruntfile showing how to set it up. This can also be combined with normal file syntax, so I'm going to add that in the PR in a minute.