Allow metaframe url paths to be relative

[dionjwa]

metapage.metaframe.src: /path-to-metaframe

resolves to <parent metapage origin>/path-to-metaframe.

This means if a metapage has associated metaframes, for example, a server listening on a metapage inputs/outputs and piping them to internal metaframes, it would be nice to not have to bake in the metaframe origin. This might also help with react router paths.