IEX Cloud API & Alpaca Pricing Source

Until now, IEX has been the primary source of data for pipeline-live. With their update to IEX Cloud, however, several of the data endpoints they provide will no longer be freely available for most use cases due to their monthly messaging limits. This update allows users to access IEX Cloud if they wish by setting an IEX_TOKEN environment variable, but also provides access to Alpaca's price data API, which does not impose messaging limits as IEX Cloud does. Please see the Readme for updated instructions on using the Alpaca pricing data source.