Preptools

* Remove the deprecated `setGovernanceVariables` and `registerProposal` commands
* Improve help messages

* Add sub-command for call network proposal by inwonkim in https://github.com/icon-project/preptools/pull/41
* Refactor for handling version information by sink772 in https://github.com/icon-project/preptools/pull/42
* Support Python 3.7 and above by sink772 in https://github.com/icon-project/preptools/pull/43

* Support new network proposals provided by governance-2.x score.
* Add applyProposal, makeProposal and registerProposal2 commands.
* Update README.md