and introducing list_dependencies_command.  (Ted, Holger)

Reading the tox.ini config file and processing it is done as one task, yet
processing the minversion is done much later, as such if there is a bug fix
in a newer version or a change that makes the tox.ini file incompatable with
older versions tox will bail with a stack trace instead of a
"Your tox is too old" message.

This moves the version check as early as it can possibly be done, the very
first value read out of tox.ini is minversion and it is immedeiatly compared
to tox.__version__.

Thanks Julien Castets for initial PR and patience.