Permalink template in page and fingerprinting

This is an experiment to support fingerprinting for page URLs. The feature would be useful for cache busting, especially when you cannot use custom Jekyll plugins in GitHub Pages.

Fingerprinting support builds upon another feature: now you can define templates per page, overriding the permalink setting in site configuration. I don't know how this aligns with the direction Jekyll's development is going, but I found this the easiest way to get into supporting fingerprinting without breaking compatibility or adding new configuration option.

Note that the fingerprint is computed from the result of page conversion (transformation). This is a must so that changes in imported Sass partials affect the outcome of the fingerprint.

I hope to hear feedback and especially like to see some kind of support for fingerprinting pages in Jekyll.