adding pre and post processing path file hooks

[narfdre]

Since we are running behind a proxy we had the need to programmatically change where usemin looks for the source file and where it outputs the destination in the tags.

We added file path pre- and post-processing hooks to accomplish this. It should be backwards compatible

[zont]

generatedUrl(name, path, file)

path is a dependency of gulp-usemin. It can not be an argument of the generatedUrl.
Arguments should be:

generatedUrl(name, file)

[trojanc]

I need this too!