Add --enable and --disable for extensions and fix --teamcity

[CharliePoole]

Fixes #1570

[stevenaw]

Thanks @CharliePoole No concerns from me.

Curious to hear your thoughts on if we should be issuing a warning or info of some kind if a user tries to "disable" an extension which couldn't be found. There would be no functional difference to the run either way, of course, but such a scenario could arise from a misconfiguration that a user may appreciate a notice about.

[CharliePoole]

Thanks for reviewing @stevenaw .

I thought about a warning and didn't do it for several reasons.

1. We don't do warnings very well unless they pertain to individual tests. In that case we give a WARNING result. But general warnings can only be displayed in the output as text, probably at the start of the run.
2. Giving a warning doesn't work very nicely with the use case where the test may be run on various machines with or without the extension and you want the result to be the same on all machines.

So, I decided to leave it out on the principle that someone with a use case where a warning would really be helpful can always ask for a change.

[stevenaw]

That makes a lot of sense. Thanks for the explanation.