A custom command can be called, and `rclone.js` CLI will look for the
module that provides the functionality.
For example, when calling the custom command `echo` below:
```sh
$ npx rclone echo arg1 --string value arg2 --boolean
```
`rclone.js` CLI will look for the followings:
- A `echo.js` in the current working directory.
- A `echo/index.js` in the current working directory.
- A `echo` module inside `node_modules`.
- A `rclone-echo` module inside `node_modules`.
The custom module just needs to export a function that takes arguments
and flags parsed from the CLI. It can either return a child process, or
a `Promise`. For a child process, its `stdout` and `stderr` are piped
to the caller process.