Support on_exit for registering callbacks in ExUnit.
Fixed reading .mix/config.exs
Merge pull request #2373 from jwarwick/cli

Fixed reading .mix/config.exs
Merge pull request #2365 from MSch/exunit-on-exit

Support on_exit for registering callbacks in ExUnit.
Move on handler functionality to its own module
Also support on_exit on setup_all
Deprecate teardown and teardown_all