Add transformFilePath option

[mxstbr]

When bundling a React app with create-react-app, it hashes the resulting bundle names, i.e. the filenames change every time.

This PR adds support for a transformFilePath option, which allows for usage with hashed file names. The example taken from the test I added:

    const plugin = new SizePlugin({
      globPattern: '**/*',
      // Remove hashes from filenames with a regex, note that this is not the most solid regex but good enough for the test
      transformFilePath: (path) => path.replace(/(.+?\.)(.+?\.)?(txt|jpg)/i, '$1$3'),
    });

[mxstbr]

CI fails due to me being on a fork I think:

Bad response: 422 {"message":"Couldn't find a repository matching this job.","error":true}

The tests pass though!