Change assetFingerprint defaults.

* 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
0.24.0