Add support for path prefixes

This PR adds support for adding prefixes to the generated paths in the mapping file.

An example use case:

**src:** assets/css/min/\*.css
**dest:** assets/css/hash/

**desired key:** css/\<file\>
**desired value:** css/\<file\>

Can be accomplished with this config:

    {
      options: {
        mapping: 'hashes.json',
        srcBasePath: 'assets/css/min/',
        destBasePath: 'assets/css/hash/',
    
        srcPathPrefix: 'css/',
        destPathPrefix: 'css/'
      },
      src: 'assets/css/min/*.css',
      dest: 'assets/css/hash/'
    }