Support complete custom response spec

[greyli]

Enhance the doc decorator to support passing a complete response spec for the responses parameter. For example:

@app.doc(responses={
    404: {
        'description': 'Custom error',
        'content': {
            'application/json': {
                'schema': SomeErrorSchema
            }
        }
    }
})

fixes #327

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 docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue.
• Add *Version changed* or *Version added* note in any relevant docs and docstring.
• Run pytest and tox, no tests failed.