Add message filtering ability

[thebigpotatoe]

Been using this library for a while and absolutely love it. The async nature is second to none above similar libraries. However I find myself writing boiler plate every time I set up a new project to filter incoming messages to achieve the correct action in the onMessage() callback.

To help improve on the library, I added the ability to filter the topic of an incoming message in the _onMessage() function when looping through each of the added callbacks from the user. This is achieved by supplying a chosen topic when adding a callback with onMessage() and a new method onFilteredMessage(). These achieve the same outcome, but one is easier to follow when reading code.

AsyncMqttClient& AsyncMqttClient::onMessage(AsyncMqttClientInternals::OnMessageUserCallback callback, const char* _userTopic = "#")' 'AsyncMqttClient& AsyncMqttClient::onFilteredMessage(AsyncMqttClientInternals::OnMessageUserCallback callback, const char* _userTopic)

I also added a shortcut for this by overloading subscribe(). By supplying a callback when adding a subscription, the callback will be added to the vector called by _onMessage() actually removing the need to supply a callback using onMessage().

uint16_t AsyncMqttClient::subscribe(const char* topic, uint8_t qos, AsyncMqttClientInternals::OnMessageUserCallback callback)

I think with these two methods and a little documentation on how to use them would make using the library a little more straightforward and far more efficient for new users when subscribing to more than one topic needing more than one callback. It will also reduce a bunch of boilerplate in a fair few of my projects and I hope a few from others.

I have tried to keep to the same formatting and style for easy integration, and have also updated all supporting files such as the examples and keywords.

[NichtEuler]

This would be great instead of having to compare 5000 strings.