Ability to pass in or override the adafruit.io hostname

[edmaher]

In the AdafruitIO_ESP8266 class, the constructor creates the mqtt and http client objects with reference to the _host, _host_port members, and uses a hardcoded value for the mqtt port
There is no ability to set or override these values in the constructor.
Ideally, there would be a second constructor that allows these to be set, or for the constructor to have additional parameters.

Other ideas are to allow the developer to use preprocessor defined values that can be set in the user's codebase, however, ability to set these at runtime is preferred.