Respect GIT_COMMITTER environment variables

[c-w]

This commit adds support for git committing environment variables. Specifically:

• If GIT_COMMITTER_NAME is set, use its value instead of the user.name git config value.
• If GIT_COMMITTER_EMAIL is set, use its value instead of the user.email git config value.
• If GIT_COMMITTER_DATE is set, use its value instead of the current timestamp.

Resolves #67