PROS-CLI 4: Upgrade Click

[omegaStag]

The CLI uses Click major version 6.7, and the most recent version of Click is on major version 8. In order to upgrade to Click's most recent version, we need to do a rewrite of a lot of the CLI, hence, PROS-CLI 4. Documentation for Click is here. Once this is done, we can add things like tab completion to the CLI. New branch develop-pros-cli-4 has been created for this purpose.

Version 6 to 7, more improvements/auto complete
7 to 8, most breaking changes here - how commands are grouped

[omegaStag]

Add tab completion: #138

[mayankpatibandla]

@ayushuk We upgraded to Click 8 in #307 can we close this?

[ayushuk]

Closing because this was done in, #138 is still a TODO.