Add footer with release date and author

[mantoni]

E.g.:

1.2.3

• Some change
• Other change

Released by Maximilian Antoni on 2018-07-03.

[mantoni]

This could work like this:

• Read $GIT_AUTHOR_NAME and if not present, skip the by ${name} part and just show the date.
• Read $GIT_AUTHOR_EMAIL and if not present, skip linking to the homepage.
• Search github for the user email with https://api.github.com/search/users?q=${encodeURIComponent(email)}&in=email, skip linking to the homepage on failure or response.items.length !== 1.
• Link to homepage using response.items[0].html_url.

If the GIT_AUTHOR_* values are not available, we could try to fall back to GIT_COMMITTER_* instead.