commands: teach arg decorator about envvar default

This makes it easier to use an environment variable as the default
value for a command line argument. Just specify the variable with the
envvar keyword argument.
tox: avoid environment values bleeding into docs (fixes #101)

The defaults for the command line flags are shown in the generated
documentation. When the default comes from an environment variable,
this can show unwanted information.

We now set _TOX_SPHINX when running sphinx from tox and the arg
decorator will use this as a signal to avoid setting the default
value.
commands: insert default value in cmdline arg help string
Merge pull request #108 from mgeisler/avoid-sphinx-envvar

Fix Sphinx problems with cmdline flags that take defaults from environment variables