snapcraft help doesn't show --shell-before, or --shell, or --destructive-mode options

[Ian Johnson]

In the snapcraft help the only options shown are --debug, --version and --help:

```
$ snapcraft --help
Usage: snapcraft [OPTIONS] COMMAND [ARGS]...

  Snapcraft is a delightful packaging tool.

Options:
  --version Show the version and exit.
  -d, --debug
  --help Show this message and exit.

```

when you can really use the default sub-command (`snapcraft snap` AIUI) options with `snapcraft` directly, so the documented options for `snapcraft snap` should also show up in the default help message. Specifically these options:

```
Options:
  --target-arch <arch> Target architecture to cross compile to
  --debug Shells into the environment if the build fails.
  --shell Shells into the environment in lieu of the step to
                        run.
  --shell-after Shells into the environment after the step has run.
  --destructive-mode Forces snapcraft to try and use the current host to
                        build.
  --use-lxd Forces snapcraft to use LXD to build.
  -o, --output TEXT path to the resulting snap.
  --help Show this message and exit.
```