Allow command-based config

[alem0lars]

Although this can be thought for multiple systems, OS X comes to my mind.
There I see tons of scripts being used for configuring the system.

So the issue mainly regards to allow to specify OS X configuration commands, like defaults instead of full configuration files.

We can maybe start from this code.

Of course we can think if it can be thought cross-platform.

Also it should be possible to implement custom commands as plugins.
See #23 for details about the plugin system.

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