Support inline $schema definitions

[davidkna]

JSON-files that follow a JSON-schema often include a reference to the schema.

{
  "$schema": "https://starship.rs/config-schema.json"
}

Please also support this for TOML files:

"$schema" = 'https://starship.rs/config-schema.json'

jsonschema seems to support doing this (#211).

[tamasfe]

This is implemented in the next version (read more in #231).