[WIP] Implement extensions API

[3y3]

Work on description is in progress

Add new configuration prop --extension <path>

Path can be absolute or relative to node_modules.

Extension is a module with default export.
Default export can implement one or more methods:

registerOptions(yargs): Promise<void> | void {}

registerTransformer(cliConfig): Promise<void> | void {}

registerIncluder(cliConfig): Promise<void> | void {}