Support asset hash

[ngs]

Fixes #6

[tisba]

This looks great! I've just checked this against our website and it works as expected.

[tisba]

I just realised, that this does not seem to work with assets outside of the image path. That might be our fault, but the asset digest logic works fine... Need to take another look.

[tisba]

I'm trying to wrap my head around another change: I also noticed that you've dropped base_url. Is there a way to ensure that attributes like og:url and og:image are full, absolute URLs now?

Reading https://ogp.me it looks like og:url needs to be URL and cannot be a path 🤔

[ngs]

Currently I'm mis-using http_prefix so will recover base_url.

[tisba]

For images actually, one can set activate :asset_host, host: "https://example.com" and this branch will do the right thing for the image. But that does not seem to work for og:url.

[tisba]

I'm not that familiar with middleman internals, but it looks like Middleman::Util.url_for(app, obj) takes an option called relative. Maybe this can be used somehow?