Show possible values in `snage create --help`

[tvrg]

The help text of snage create should show the possible field values for all fields. E.g. if components is of type string with enum values [server, ui, cli, api, config, docs] and has list: true, snage create --help should display something like this:

$ snage create --help
snage create

Create a note.
...
  --components   DESCRIPTION [one or more of server, ui, cli, api, config, docs]
...

[tvrg]

It would also help if the help text included a hint whether or not a field is optional.