Allow passing through arbitrary arguments to different commands

[victorb]

As more things like #148 pop-up, where we'd like to customize running the different tools in different environments, it'd be helpful if we could find a way to pass through arguments and parameters to tools without having to change aegir itself.

For example, would be helpful to be able to pass --bail to jest which is run via aegir test but without having to modify the aegir command to add all the jest options.

[victorb]

Applicable both for running via CLI and via .aegir.js

[dignifiedquire]

How do you expect that to work when calling something like aegir test which calls two very different tools in the background

[victorb]

Hm, spitballing:

aegir test --jest-args "--bail" --mocha-args "--timeout 500"

or something like that.