Add pre-write transformation

[mkahvi]

Implements #56

Tested with the following transformer:

transformSerialized: async (content, { filename } = {}) => {
  return prettier.format(content, { ...prettierConfig, filepath: filename });
},

[Fyorl]

I have modified this to pass the full filename in the serialization context since it avoids having to pass extra arguments. You can extra the last portion with path.basename if needed.

[mkahvi]

@Fyorl I think I had it not use that because you couldn't nicely determine the proper path of the file with that. It included a lot of spurious details that were not part of the project's pathing. At least in my testing it had really weird paths compared to what the PR separately used.

[Fyorl]

@Fyorl I think I had it not use that because you couldn't nicely determine the proper path of the file with that. It included a lot of spurious details that were not part of the project's pathing. At least in my testing it had really weird paths compared to what the PR separately used.

From just looking at the code, as far as I can tell, in all cases where you passed filename: name or filename: embeddedName, etc. just a few lines up we have filename = path.join(dest, name). So it should always be possible to extract the name in that case because path.basename(path.join(dest, name)) === name.