Implement the help command

There is an initial implementation to display the help for the rover --clone. This issue extends the implementation to the other commands available

Proposed approach:

rover --help to display all commands available rover --clone --help to display the clone command rover --landingzone -h or rover -lz --help to display the landingzone help command

When an a value is missing for a given attribute the rover displays the example and the detailed explanation of the attribute. For example rover --clone-branch requires to set the name of the branch. The rover will fails and display the examples and the detailed explanation of the --clone-branch