[RFE] "Blacklist" flag

[stephenfin]

This might be related to #58 but I'm not sure.

I'd like to integrate tox-docker into my project. I currently have [tox] envlist like so:

envlist = pep8,docs,py{27,34}-django111,py{35,36}-django{111,20,21,22}

I want to use tox-docker so I can optionally run a docker container to provide the database needed for these tests. The natural way to do this is using factors like so:

envlist = pep8,docs,py{27,34}-django111-{mysql,postgres},py{35,36}-django{111,20,21,22}-{mysql,postgres}

However, when I do this, tox-travis attempts to run these additional containers which doesn't work out too good.

I'd like to blacklist any environment containing the mysql or postgres factor. Does anything like this exist and, assuming not, could we get it?