common: Add pre and de emphasis, if the radio does not support it

Connect to the oscillator of the transmitter - good girl!
You need to enable pre emphasis in this case.

Connect to the discriminator of the receiver - good boy!
You need to enable de emphasis in this case.

src/common/emphasis.h

@@ -0,0 +1,17 @@
+typedef struct emphasis {
+	struct {
+		double last_value;
+		double factor;
+		double amp;
+	} p;
+	struct {
+		double last_value;
+		double factor;
+		double amp;
+	} d;
+} emphasis_t;
+
+int init_emphasis(emphasis_t *state, int samplerate);
+void pre_emphasis(emphasis_t *state, int16_t *samples, int num);
+void de_emphasis(emphasis_t *state, int16_t *samples, int num);
+