Implement help as a plugin to show detailed command help

Branching off from #101. It would be cool to implement `shx help` as a ShellJS plugin, and let this show help output at a finer granularity:

```sh
$ shx help ls
Usage: ls [option]... [file]...
  -l  use a long listing format
  ...
$ shx help cp
...
```

Not sure if there's a good way to do this right now without parsing ShellJS docs. But we could certainly support 