Add support for Metacello configurations

[fniephaus]

Some projects (e.g. [1]) use Metacello configurations instead of baselines and therefore, we should add support for configurations to smalltalkCI.
This is related to #21.

[1] https://github.com/frankshearar/Control

[fniephaus]

@frankshearar: I have added support for Metacello configurations to smalltalkCI today.

Since we are still discussing how to configure Smalltalk builds in the future, a Metacello configuration can only be set via command line parameter at the moment.
So once we have a solution you should probably update your project's configuration once more.

The following should now work for your Control project:

language: smalltalk
sudo: false
smalltalk:
  - Squeak-trunk
  - ...

script: $SMALLTALK_CI_HOME/run.sh --mc-config "Control"

[frankshearar]

Thanks very much @fniephaus - works like a charm!