Mapping base URI

[namedgraph]

Issue type: 🦄 Feature

Description

Base URI that the URIs (and URI templates) in the mapping would resolve against

Why it is useful

Right now we use YARRRML "templates" with placeholders in the URI values in order to customize them.
I think base URI and relative URI resolution would allow us to solve it in a much cleaner way.

Existing features it breaks

None that I know of.