Implement per-field stringifier

[javcasas]

This implements a per-field stringifier, so that you can customize how each field is written. Effectively, it's the opposite of the cast option.

[greguz]

Hi Javier, nice addition! I just need to ask you to remove any package.json or version edit from the PR.

I will bump the package version in a different PR, just before publishing the package (right after you PR). Thanks.

[greguz]

Also, please run npm run lint and npm run test to test that your changes are OK (your PR fails the lint script).

[javcasas]

@greguz Requested changes done.