Added stripPrefix, prependPrefix, and processPath

This PR includes options that give `karma-html2js-preprocessor` feature parity with `karma-ng-html2js-preprocessor` for non-Angular use cases.

The following options have been added:

* `stripPrefix` - Remove this prefix from the file path
* `prependPrefix` - Prepend this prefix to the file path
* `processPath` - Custom function that accepts the `filePath` and transforms it as necessary

`processPath` is equivalent to  `karma-ng-html2js-preprocessor`'s `cacheIdFromPath` option. I'm not sure on the naming here, I'll gladly change it if there is a better name that follows Karma's naming conventions better.