--format options undocumented

[evmar]

🐛 Bug Description

The --format flag's possible values aren't described anywhere.

twiggy version: twiggy-opt 0.6.0

🌍 Test Case

Upload the test case and link to it here. For example, a .wasm file that
Twiggy fails to parse.

👟 Steps to Reproduce

$ twiggy help monos
...
    -f, --format <output_format>         The format the output should be written in. [default: text]

$ twiggy monos --format help mywasm.wasm
error: Invalid value for '--format <output_format>': Unknown output format: help