Add archival feature tagging on feature finish

When finishing a feature, add a tag with the feature name, prefixed with
the configured archive prefix. Example: a feature named foo would be
archived as archive/feature/foo. The tag is set immediately behind the
merge commit

Having such tags eases resumption of feature branches that are
prematurely finished. They also mark the end of each feature branch in a
way that's easier to find than searching for the final merge commit.

Add a flag to feature finish, -A, that enables archive tagging if it is
not on by default in the config.

Add a new boolean configuration option, gitflow.options.archivefeatures,
which if set true, archives features by default, without specifying -A
on the command line.

Add a new prefix in git flow init: gitflow.prefix.archivetag, with
default archive/

Add a summary message if branch was archive tagged.
Defend against missing prefix for archive tag
Bumped version number to 0.4.1-true-109-gd80b3a8+archive-tagging
Prevent error message on flow version command

The version subcommand lacks an init function. This defends against it.
Bumped version number to 0.4.1-true-111-g33b2f93+archive-tagging
Add <object> argument to tag command for archive tagging

Without the third argument, the tag would apply to whatever was checked
out, not necessarily the tip of the feature branch.
Bump version to 0.4.1-true-117-g3a6451e+archive-tagging