Flask-cli

- Adds a --version switch to Flask CLI.

- Updates help to `> set FLASK_APP=hello.py`.

- Adds plugin support to the CLI.

- Updates docs to the new CLI patterns.

- Removes Python 2.6 and adds Python 3.5 to Tox configuration to match
Travis config.

- Implements simplified CLI interface.

- Werkzeug should not block propagated exceptions from Flask.

- Fixes issue with the parameter passed to the CLI's AppGroup.

- Adds shell context processor example.

- Fixes missing import in Python example in README.

- Fixes broken tests.

- Adds support for specifying shell context processors.

- Fixes bug with shell command due to missing support for shell context
processors.

- Adds FlaskCLI extension to support app.cli.

- Changes default imports from flask_cli.cli to flask_cli and relies on Flask's
classes when v1.0 is installed.

- Initial public release