Add option for the user to define new filter resolver repositories

[Nusiq]

Use new command: regolith repo

• regolith repo - lists installed resolvers with their indices. Sth like:

  Your installed repositories:
  - [1] github.com/Bedrock-OSS/experimental-regolith-resolver
  - [2] github.com/SecretOrganisation/super-secret-regolith-resolver
  - [3] github.com/something/something-regolith-resolver
  

• regolith repo --add <url> adds new filter resolver
• regolith repo --remove 2 removes the second item from the regolith repo list

[Nusiq]

Closed with #225