For crates with binaries, allow writing of --help docs to README

[clbarnes]

Possibly best supported in the template file, something like

# {{crate}}

{{readme}}

## Binaries

### `my_bin_name`

{{bin:my_bin_name --help}}

### `my_other_bin`

{{bin:my_other_bin -h}}

which would produce something like

# my_crate

Some information I've totally written in my crate docs.

## Binaries

### `my_bin_name`

```
usage: my_bin_name [-h] arg [arg ...]

positional arguments:
  arg

options:
  -h, --help            show this help message and exit
```

### `my_other_bin`

```
usage: my_other_bin [-h] arg [arg ...]

positional arguments:
  arg

options:
  -h, --help            show this help message and exit
```

This would allow somewhat-arbitrary code execution which isn't ideal but this is a really useful feature to have when it comes to self-documentation. My current solution is a python script which pipes text into a labelled code block: https://pypi.org/project/pipe2codeblock/