IRQManager: implement callback based method for custom irq handlers

[facchinm]

Could fix #310

I'm not 100% convinced about the freedom <-> ease of use trade off this API should have, in the meantime it's leaning towards the freedom (since the expected audience is not the beginner).

@delta-G what do you think?

Copying here the test sketch I added to the code

void irq_callback() {

}

bool config_my_funky_peripheral(unsigned int* last_interrupt, volatile uint32_t *irq_ptr, void* config) {
  funky_peripheral_cfg_t* cfg = (funky_peripheral_cfg_t*)config;
  *(irq_ptr + *last_interrupt) = (uint32_t)irq_callback;
  cfg->interrupt = *last_interrupt;
  *last_interrupt++;
  return true;
}

funky_peripheral_cfg_t funky_cfg;

void setup() {
  // put your setup code here, to run once:
  IRQManager::getInstance().addCustomPeripheral(config_my_funky_peripheral, &funky_cfg);
}

[delta-G]

I could use that, but it's a little scary letting the general public have last_interrupt_index like that. May I propose an alternative that keeps the code inside IRQManager? All we need is a generic config struct that has an IRQn_Type, a priority, and an elc_event_t. The user passes in the even type and the priority and IRQManager fills in the irq number.

I've can put that code together real quick and push another PR and we can compare.