added install and update support for homeshick

This pull requests implements the option to support an install and update file which can be executed either manually via homeshick install CASTLE or homeshick update CASTLE or automatically during clone, pull and refresh actions.

This allows to run scripts to make further changes which are not possible by symlinking alone.

Question: Would it make sense to move the install and update invokement for clone, pull and refresh in front of the original functions?