Support for writing extra fields.

[acg]

This change lets callers add tagged extra fields to central directory records. For compatibility with yauzl, the shape of extraFields is the same: an Array with each entry of the form {id: id, data: data}.

[andrewrk]

OMG my old co-worker just made a pull request to my best friend ❤️

[thejoshwolfe]

Hi @acg . Sorry for the delayed response. (wow it's been 5 years.) Thanks for the high-quality PR! updating docs and tests is definitely appreciated.

I'm curious which extra fields you want to include in the zip file, or if you want to perhaps preserve all of them that exist in an existing zip file? If the latter, this sounds similar to #38 and #76 .

If you're interested in specific extra fields, the test changes suggest that perhaps the 0x7875 Info-ZIP UNIX (newer UID/GID) extra field is what you want. This is similar to #70 .