Add support for allowed values for flags

[DannyBen]

Closes #65

---

TODO:

• Implementation
• Second opinion test
• Update README
• Add example
• Implement specs

---

This PR allows defining allowed: [value1, value2] for flag items

# bashly.yml
flags:
- long: --user
  short: -u
  arg: NAME
  allowed: [user, admin]
  # required: true
  default: user
  help: "User name"

This will:

Exit with an error unless the provided value is in the whitelist:

$ ./login --user asd
--user must be one of: user, admin

Show the allowed default values in the usage text:

$ ./login --help
...
Options:
  --user, -u NAME
    User name
    Allowed: user, admin
    Default: user
...

As usual, output strings are configurable.