Add instructions and a hook to call core-validate-commit from
git commits.

Depends on nodejs#11

Known issues:
- leaves ${TMPF} around (temporary file)
- probably will reject `fixup:` type commit logs
- may be too strict if someone is just doing a simple
'work in progress' kind of local branch

Larger improvement:
- fold the entire hook script into a mode, and/or an alternate
script entrypoint into core-validate-commit. This would drop
requirements on sh, sed, grep…

Fixes: nodejs#12