Support for subcommands as distinct binaries

It would be nice to be able to create a binary such as `fresh-open` which will be executed when you run `fresh open [path]`.

`fresh --help` and tab completion should recognise these commands. Ideally there’d be a way to delegate subcommand option completion as well.

We should create an example project which can be sourced with:

``` sh
fresh freshshell/fresh-open bin/fresh-open --bin
```

- [x] Functionality
- [x] Documentation (wiki)
- [x] Completion
- [x] Tests
- [x] Documentation (README)
- [x] `fresh --help`

**Note:** `fresh --help` should use `fresh commands`.