Add hello application for chapter 1

demos/hello/.flaskenv

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+FLASK_ENV=development

demos/hello/app.py

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+# -*- coding: utf-8 -*-
+import click
+from flask import Flask
+
+app = Flask(__name__)
+
+
+# the minimal Flask application
+@app.route('/')
+def index():
+    return '<h1>Hello, World!</h1>'
+
+
+# bind multiple URL for one view function
+@app.route('/hi')
+@app.route('/hello')
+def say_hello():
+    return '<h1>Hello, Flask!</h1>'
+
+
+# dynamic route, URL variable default
+@app.route('/greet', defaults={'name': 'Programmer'})
+@app.route('/greet/<name>')
+def greet(name):
+    return '<h1>Hello, %s!</h1>' % name
+
+
+# custom flask cli command
+@app.cli.command()
+def hello():
+    """Just say hello."""
+    click.echo('Hello, Human!')