Use of cobra package to refactor CLI

[arnabghose997]

Closes #160

NOTE: Must be merged only after PR #190 is merged

This PR intends to refactor the existing CLI by using the cobra CLI Golang package. Refer here for details of cobra package.

Refer here for the CLI documentation

Following are key highlights of this PR:

• The Command struct has been renamed to CommandConfig in order to avoid confusion with the Command struct of cobra package.
• Commands have been grouped in a hierarchical manner. Refer here for more details.
• Command flags are prefixed with double hyphen (--) instead of single hyphen (-). Flag names are have not been changed. Ever command now displays only those flags which are associated with their operation. addr and port are the only Global flags because of their usage in almost every command.
• Following existing commands have been removed:

Comand	Reason
setupDID	Implemented logic is exactly same as RegisterDID (Refer here)
getTokenBlock	No Implementation (Refer here)

• path.Join() has been used, instead simple + operation, for cleaner file path concatenation.
• In tests/node/command.py The -grpcPort flag has been removed from commands wherever it is not required.
• Changes in root README.md