Add pre/post command on test

[lordofthejars]

In some situations it may be useful to run pre or post operations on test phase. I am thinking in next scenario.

Project A has a Project A-AT which contains the acceptance tests for project A. So before releasing a container you may want to get Project A-AT source code, run its own captain file to generate Project A-AT container (which may contain for example phantomjs) and then in test phase itself running both containers to execute the tests.