Make "tox -e flakes" pass

and introducing list_dependencies_command.  (Ted, Holger)

Process minversion before all else

This adds a test to address the second example in issue #307, the
first example is already addressed.

Closes issue #307

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__.

Use crossonly to stop substitution immediately after the section is
substituted, the values run through substitution again afterwards, and
this will clean off {posargs} and any other substitutions that need
to be made.

Test command substitution from other section with {posargs} in use.

Issue #279