Added support for Git commit information.

If the source directory is a Git repository, basic information on the
last commit a Markdown file was changed in is passed to the page
template when converting it.

Currently the only information that's exposed is:
- ID
- Short ID (the first 7 characters of the ID)
- Timestamp
- Committer name
- Committer email address
- Short commit message

That's enough, for now.