Download data from the Alpha Vantage API (<https://www.alphavantage.co/>). Alpha Vantage is a RESTful API which provides various financial data, including stock prices and technical indicators. There is documentation for the underlying API available here: <https://www.alphavantage.co/documentation/>. To get access to this API, the user needs to first claim an API key: <https://www.alphavantage.co/support/>.
