Add support for the submodules

[dl1998]

Add support for the Git Submodules, every submodule shall support the same operations as the main repository, ideally initialize git repository for every submodule and store it in the submodules variable in the top level repository.

Additionally submodule command extends functionality of the repository managing, git submodule command options shall be supported as well.

Links:

• Git Submodules Description
• git-submodule command documentation