Add configuration file support

The configuration file allows the user to change the appearance of the
prompt without changing the script proper (so that future updates to the
script can be seamless).  It also provides a central place for such
configurations.
Add tag to the prompt

By default, the tag will only be shown if it points directly to the
current HEAD; a configuration option is provided to allow showing the
tag information at all times (when no tag points directly to HEAD, the
closest "parent tag" as determined by "git describe" will be shown).