Add caching.

mypy.ini

 [mypy]
 files = whatformat/**/*.py
+
+[mypy-flask_caching.*]
+ignore_missing_imports = True

requirements.txt

 python-dateutil==2.8.1
 Flask==1.1.1
+Flask-Caching==1.8.0

whatformat/app.py

 """Flask app for What Format."""
 
 from flask import Flask, jsonify, render_template, request
+from flask_caching import Cache
 from whatformat.util import whatdateformat
 
 # pylint: disable=invalid-name ; These variables are standard for Flask.
 app = Flask(__name__)
+cache = Cache(app, config={'CACHE_TYPE': 'simple'})
 
 
 @app.route('/')
+@cache.cached(query_string=True)
 def landing():
     """Render the homepage."""
     date_string = request.args.get('date')
@@ -15,6 +18,7 @@ def landing():
 
 
 @app.route('/date')
+@cache.cached(query_string=True)
 def date():
     """Return date format strings matching date_string."""
     date_string = request.args.get('date')