Expose commit hook

[davidisaaclee]

Checklist

• I grant to recipients of this Project distribution a perpetual,
  non-exclusive, royalty-free, irrevocable copyright license to reproduce, prepare
  derivative works of, publicly display, sublicense, and distribute this
  Contribution and such derivative works.
• I certify that I am legally entitled to grant this license, and that this
  Contribution contains no content requiring a license from any third party.

I made this PR largely by copying #210 and trusting tests. I just now switched a project to use my fork, and commit_hook() appears to be working as expected.

I think we could add the rollback hook very easily with this work, but I didn't need that. If you want that feature here, I'd be happy to implement it.

I committed a build made using Emscripten 3.1.70.

[rhashimoto]

Thanks for the PR! Found the commented issue on reading through your changes.

If you can please rebuild dist/ with EMSDK 3.1.61 that will make it easier for me to verify your binaries.

[rhashimoto]

I can build byte-for-byte identical binaries.

Just fix these cosmetic nits and I'll merge.