Allow dynamic disable of plugin

[stdunbar]

I'd like to be able to selectively disable the download of all drivers. Use case is running against Browserstack. In that case there is no need to download anything locally. Perhaps a system property (-D on the command line) or an environment variable.

[stdunbar]

Of course, I can just set up a profile to disable it. Closing issue.

[Ardesco]

beat me to an answer :)