Create a CLI setup script

[blueridger]

Create a script that prompts the user for the required configurations for an instance, including:

• Plugins used (sourcecred.json)
• Tokens for each plugin (.env)
• Grain distributions (grain.json)

Additionally, it can call plugin-specific sub-scripts that build the plugin configs.

A bash script would be the obvious choice, though there are other options. It would be really nice to utilize the new Instance interface for read/write (sourcecred/sourcecred#2764).

[KuraFire]

Can I be involved in this to help design the script flow & language?