Support selecting host to use when creating a gist

### Support selecting host to use when creating a gist

When logged into github.com and an enterprise version of Github, when I go to create a gist, it defaults to creating the gist on github.com. There is no simple way to specify which host to use when creating a gist.

### Proposed solution

I suggest adding a `--host` and `-h` parameter which would allow one to specify which host to use when creating the gist. This will make it easy to select a host on a command by command basis. The host specified should match one listed in`$HOME/.config/gh/hosts.yml`.

### Additional context

For Linux users, the workaround today is to temporarily set the `GH_HOST` variable before running the command, like this:
```
GH_HOST=github.my-company.com gist create ...
```

If this feature is not implemented then I suggest the documentation should be updated to document the workaround above.