gw recipe: pre and post functions

[danwos]

During the registration of a recipe, the developer shall be able to register pre- and post-functions for the recipe build.
These functions get executed, if a build for the recipe gets executed.

use cases for pre-functions:

• Check if installation folder is ready
• Prepare installation folder (e.g. set permissions)

use cases for post functions:

• Check if everything got installed correctly
• Set permissions
• Add additional files

Build gets only interpreted as success, if pre and post functions have been executed without throwing an exception or returning "False".

[danwos]

Have a look at https://cookiecutter.readthedocs.io/en/latest/advanced/hooks.html

Maybe these mechanism can be used to invoke the pre/post functions.