Update init command to load existing configuration and prompt defined target paths as defaults

[andreiled]

Problem Statement

If/when in future I add new configuration path options to the global auto-download configuration (stored in the user's directory) requiring to re-run the init command to initialize them, then it would be a bad user experience to have users re-enter all other paths already defined in the config.

Proposed behavior

The init command should load existing configuration if it exists and propose already defined paths as default values for the corresponding prompts. This should allow users to quickly skip through the prompts confirming their previous choices until they get to the point where init asks to specify the new paths.

Alternatively, or additionally, consider updating existing configuration using new answers while preserving any customizations that may have been manually made to the existing configuration instead of re-generating the whole configuration document from scratch.