Refactored to accept a `formatFn` for logging.

Addresses [#1](https://github.com/ben-eb/gulp-bytediff/issues/1).

Also, fixed `file.contents` size calculation. `Buffer.byteLength()` assumes
`'utf-8'` encoding, which produces inaccurate results for files that have
been compressed (for example, with [gzip()](https://github.com/jstuckey/gulp-gzip)).