Further Polygon stream improvements

In this release, several code issues with websocket reconnection are fixed. The ability to specify a list of symbols for a websocket handler method to listen (only) for has been added. (To use this, simply specify a list of symbols as strings after the channel pattern in your on() decorator.) Additionally, an unsubscribe function is provided for the Polygon stream, as well as a configuration option for specifying a non-Alpaca Polygon key in the environment variable POLYGON_KEY_ID.