Add customizable email footers.

The email-footer can be customized by using the following placeholders: %USER_NAME%, %USER_LASTNAME%, %USER_EMAIL%, %USER_LOGIN%, %USER_CF_...% for all user custom fields.
Footers are no longer appended to the first replies. Upgrade to the new version appends the footer value to the first reply setting, so users don't need to configure this manually.
Fixes #52.