load existing manifest directly from file 

this addresses sindresorhus/gulp-rev#59 and sindresorhus/gulp-rev#55.

i personally would like to read an existing rev-manifest file as part of the rev.manifest() call instead of injecting the existing manifest file into the stream of asset files:
 - these are two types of files, assets and meta-data
 - appending to existing files is broken and needs workarounds
 - its hard to read and understand

this pull request reads an existing rev-manifest.json file directly on the call to `rev.manifest()`, this changes the default behaviour to use an existing manifest and append changes onto it. if no manifest file exists, its created.