feat: multiple db support

[damphat]

# list all databases, and mark the current database with the '*'
kvuse
    * default-db
      test-1
      todo-app

# switch to todo-app
kvuse todo-app

TODO

• [ ] kvuse switch the current database
• [ ] kvuse print all database if there no parameter supplied to the kvuse command
• [ ] kvset create the database if it not exists
• [ ] kvclear should also remove database
• [ ] kvlist should print current database name at the top?
• [ ] how to go back to default-db

Questions

• Q: how to see current database?
• A: kvuse
• Q: how to list all databases?
• A: kvuse
• Q: how to create database?
• A: kvuse <db-name>
• Q: how to delete a database?
• A: kvuse <db-name> then kvclear