Add support for hardware Sigma Delta generator

Provides a simple interface to attach/detach pins to the sigma-delta generator, and get/set the 2 parameters prescaler & target.  Working example that fades the onboard LED is provided.  Code and sample by @stefaandesmet2003.

libraries/esp8266/examples/SigmaDeltaDemo/SigmaDeltaDemo.ino

@@ -0,0 +1,48 @@
+/* Any copyright is dedicated to the Public Domain. */
+
+#include "sigma_delta.h"
+
+void setup() {
+
+  Serial.begin(115200);
+  pinMode(LED_BUILTIN, OUTPUT); // blinkie & sigma-delta mix
+  uint32_t reqFreq = 1000;
+  uint32_t realFreq;
+
+  realFreq = sigmaDeltaSetup(0, reqFreq); // chose a low frequency
+
+  Serial.println();
+  Serial.println("Start Sigma Delta Example\n");
+  Serial.printf("Frequency = %u\n", realFreq);
+
+}
+
+void loop() {
+
+  uint8_t duty, iRepeat;
+
+  Serial.println("Attaching the built in led to the sigma delta source now\n");
+  Serial.printf("Current duty = %i, prescaler = %i\n", sigmaDeltaRead(), sigmaDeltaGetPrescaler());
+  sigmaDeltaAttachPin(LED_BUILTIN);
+
+  Serial.println("Dimming builtin led...\n");
+  for (iRepeat = 0; iRepeat < 10; iRepeat++) {
+    for (duty = 0; duty < 255; duty = duty + 5) {
+      sigmaDeltaWrite(0, duty);
+      delay(10);
+    }
+
+    for (duty = 255; duty > 0; duty = duty - 5) {
+      sigmaDeltaWrite(0, duty);
+      delay(10);
+    }
+
+  }
+
+  Serial.println("Detaching builtin led & playing a blinkie\n");
+  sigmaDeltaDetachPin(LED_BUILTIN);
+  for (iRepeat = 0; iRepeat < 20; iRepeat++) {
+    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
+    delay(500);
+  }
+}