Config customisation

[SStorm]

Some restructuring of the code to make it easier to override some of the definitions (especially statically allocated sizes for strings etc.).

This is achieved by optionally specifying a STUNLIB_CUSTOM_CONFIG preprocessor flag, i.e. -D STUNLIB_CUSTOM_CONFIG="\"my_config.h\"" which gets included instead of the default one. Would like to hear your thoughts on this approach. A potential alternative might be something like:

#ifdef STUNLIB_CUSTOM_CONFIG
#include "stunlib_config_custom.h"
#else
#include "stunlib_config.h"
#endif

Also, changed u_int16_t to uint16_t for better compatibility, hope you don't mind.

On a positive note, this now happily runs on a esp32 SoC with 400kb of memory ;-)

[coveralls]

Coverage remained the same at 82.594% when pulling 4146c3e on imont:config_customisation into 36cc9dd on NATTools:master.