Support referencing external copies of static resources in html report

[nedbat]

Originally reported by Tom Prince (Bitbucket: tomprince, GitHub: tomprince)

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When generating multiple html reports (for example, with buildbot), it would be nice to be able to use external copies of the static resources, so they can be cached, for example.

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/233