making the library cross platform to be abe to be used on native buids

[rOzzy1987]

I'm usingg this library for a project where I need to parse a pretty comlpex JSON object. Testing my custom listener was a nightmare on the device itself, bt since it relies on Arduino's String implementation I couldn't build an x86 appication of it to debug step by step.

Since the concept is very good (and I need it because the uC's few kB of RAM isn't enough to even store that particular JSON) I thought I'd add some macros and whatnot to make it easier in the future. this way it's only a few lines of config in Platformio to set up a native environment to debug.