Improve metadata types

[patricoferris]

Sesame should ship with some basic types to cover metadata descriptions. In particular, there should at least be an StaticImage.t and a Date.t. In the future it would be neat if Sesame could take a RemoteImage.t make a HTTP request, cache it and then treat it as any old image. Tbh this is more or less the same with any data-type that is ultimately at the moment an Fpath.t.

With these metadata types it makes it much easier for the user to write code for say an index page because they could actually do something like sort by date out of the box, rather than implementing it themselves.