Improve assetFingerprint defaults

The enableAssetFingerprint feature now behaves differently, and is much more robust as a result. This may be a breaking change if you're currently using the feature in a non-trivial way:

• Remove non-fingerprinted files by default. Feedback suggested keeping
  them around only ever resulted in errors being missed.
• Do string replacement of logical asset paths to fingerprinted asset
  paths against a set of known file extension (starting small with html,
  css, js, and xml). Override with files.assetFingerPrint.textFiles prop
• Add images & webfonts to the default set of fingerprinted asset
  (this was always desired but not feasible without string substitution)
• Simplify task loading by placing assetFingerprint task at the very
  end, instead of slicing it before the pages (another ramification of
  the string replacement strategy)
• Stop mutating the pages' context object with fingerprinted URLs,
  instead (you guessed it), lean on the string replacement feature of
  grunt-asset-fingerprint