input: Write a python function to create a Flask app with a counter feature.
  assert index() == render_template('index.html', counter=0)
  assert increment() == render_template('index.html', counter=1)
  assert reset() == render_template('index.html', counter=0)
output: from flask import Flask, render_template
  
  app = Flask(__name__)
  counter = 0
  
  @app.route('/')
  def index():
      global counter
      return render_template('index.html', counter=counter)
  
  @app.route('/increment')
  def increment():
      global counter
      counter += 1
      return render_template('index.html', counter=counter)
  
  @app.route('/reset')
  def reset():
      global counter
      counter = 0
      return render_template('index.html', counter=counter)