Multi-target versions of Stream::find() and Stream::findUntil().

Sometimes, you want to read / parse incoming stream data until you find one of two (or a few) different strings. This is particularly useful when dealing with Serial-based (or other Stream based) APIs, like the Quectel M10 GSM module or the ESP8266 wifi module.

It's not entirely clear to me what the best API / implementation would be, since there are a variety of use cases / needs. For example, with those modules, you may want to look for either an okay message, an error message, or an unsolicited event that's not in reply to your previous command.