Add support for needy flags: flag.needs

[DannyBen]

Adds support for cross-dependency between flags for cases when some flags are required only if other flags were provided.

Discovered in #542

Todo

• Initial pilot
• Confirmed implementation
• Example
• Tests
• Update bashly YAML/JSON schema
• Update strings JSON schema
• Update bashly docs command
• Apply to usage help
• Apply to mandoc / markdown generator
• Specs pass
• Update doc site

Sample bashly.yml

name: cli

flags:
- long: --add
  short: -a
  arg: alias
  help: Alias to add
  needs: [--command]
- long: --command
  short: -c
  arg: command
  help: Command to add
  needs: [--alias]