Add EnumChoice type

[dzcode]

• Add EnumChoice ParamType that inherits from the Choice type. It can take an enum.Enum and use it's keys or values as options that can then be mapped back to the enum itself.
• Allows enum keys or values to be used as the choices

Example Usage:

    class MockEnum(Enum):
        foo = 1
        bar = 2
        baz = 3

    @click.argument("method", type=click.EnumChoice(MockEnum))
    def cli(method):
        click.echo(method)

Fixes #605

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[dzcode]

Closing in favor of #2210

[davidism]

Sorry for the duplicated effort, this PR looks good too.