Add arbitrary JSON tooling to CLI

[carolinebridge]

Currently if the CLI isn't updated with tooling to add specific properties to the config there's no way other than manually editing the config file to add those properties. For users who aren't comfortable manually editing JSON, we should add this capability.

[carolinebridge]

As per meeting discussion:

• instead of making it a part of our CLI tooling, add documentation in the CLI README or in the JBrowse docs such that a user can use node / cmd line to accomplish the task of adding arbitrary JSON to a config file