Specify what to hash

[opatut]

(Originally posted at webpack-contrib/file-loader#60)

I'd like to be able to specify what to mangle through the hash function in order to get the [hash]. In my specific case, I want to hash [path][name]. I would like to have an additional parameter in the [hash] brackets that specifies a pattern (to be interpolated with interpolateName) for what to hash.

Rationale: we're building different versions that are served based on the request origin, but using the same js bundle (and server side code!), but with different file contents (e.g. different images at logo.png). If we were to hash the content, the different file had a different hash, and we could not map it from the one the js bundle requested. We don't want to use [path] in production either because it gives away our file structure. Hence we'd like to hash the path.

I also want to to be able to add two different hashes, one using the path, one using file content, to the generated name. We need this for the version suffix to trick the browser cache, i.e. generate [hash:[path]]/[name]-[hash:6].[ext].

[sokra]

see comment in PR