Implements custom commands

[searls]

Users can now specify custom commands. This had been documented previously as using a : separator between module name and command in the string, but it was not implemented. Additionally, that strategy wouldn't have worked for git-repo-URLs (as a protocol or port would contain a : before the name was complete). So instead, I created a simple object format (with properties name, repoUrl, and command) that can be specified alongside strings.

I did my best not to editorialize on style or do any other refactors in the process, so even though this is a significant change, I tried to make it as non-disruptive to you and your style as I could.

A couple notes:

• The module was previously splitting on : for non-URL module names, but since it had no useful effect previously, I doubt anyone will be broken by the fact I removed it in this commit
• npm install https://github.com/testdouble/testdouble.js works, so perhaps the custom work with ggit isn't necessary? I only glanced at it.
• A default timeout of 10 seconds seems much too short. I wasn't able to get much at all to finish in 10 seconds and it seemed like an unnecessary constraint placed by the module on users when a longer install doesn't suggest a failure
• It's pretty clear that dont-break doesn't delete test folders, at least not when things fail. (oops, this is a dupe of Delete install folder on done #33)

[bahmutov]

I agree with the format - that is something I wanted to do for a while.
I am also thinking the timeout could be a property per dependent project (or using default timeout, which I just increased to 3 minutes). Maybe rebase it against the latest code that has your 0.12 support branch merged?

[searls]

Because of all the other changes being merged and the amount of impact in the second of my commits, I just had to do a merge instead of rebasing. In any case it should be clean to merge at this point

[bahmutov]

@searls is this still relevant? We have just merged a few pull requests that do custom test command