Allow to couple/uncouple CE pin (#23)

Allow the specific display instance to be controlled by another
instance, which has been initialized with the same pins except the CE one. 
Enables having multiple displays on the same data, clock and reset pins.

src/Nokia_LCD.cpp

@@ -112,6 +112,15 @@ void Nokia_LCD::begin() {
     sendCommand(0x0C);  // Set display control, normal mode.
 }
 
+void Nokia_LCD::couple() {
+    digitalWrite(kCe_pin, LOW);
+    mCoupled = true;
+}
+void Nokia_LCD::uncouple() {
+    digitalWrite(kCe_pin, HIGH);
+    mCoupled = false;
+}
+
 void Nokia_LCD::setContrast(uint8_t contrast) {
     sendCommand(0x21);             // Tell LCD that extended commands follow
     sendCommand(0x80 | contrast);  // Set LCD Vop (Contrast)
@@ -291,7 +300,9 @@ bool Nokia_LCD::send(const unsigned char lcd_byte, const bool is_data,
     digitalWrite(kDc_pin, is_data);
 
     // Send the byte
-    digitalWrite(kCe_pin, LOW);
+    if (!mCoupled) {
+        digitalWrite(kCe_pin, LOW);
+    }
     if (kUsingHardwareSPI) {
         constexpr uint32_t kSPiClockSpeed{F_CPU / 4U};
         SPI.beginTransaction(SPISettings{kSPiClockSpeed, MSBFIRST, SPI_MODE0});
@@ -301,7 +312,9 @@ bool Nokia_LCD::send(const unsigned char lcd_byte, const bool is_data,
         shiftOut(kDin_pin, kClk_pin, MSBFIRST, lcd_byte);
     }
 
-    digitalWrite(kCe_pin, HIGH);
+    if (!mCoupled) {
+        digitalWrite(kCe_pin, HIGH);
+    }
 
     // If we just sent the command, there was no out-of-bounds error
     // and we don't have to calculate the new cursor position

src/Nokia_LCD.h

@@ -231,6 +231,16 @@ class Nokia_LCD {
      */
     void setDefaultFont();
 
+    /**
+     * @brief Allow the specific display instance to be controlled by another
+     * instance, which has been initialized with the same pins except the CE one. 
+     * Enables having multiple displays on the same data, clock and reset pins.
+     * @example Allow `lcd1` and `lcd2 to both display the same as `lcd3`
+     * @example `lcd1.couple(); lcd2.couple(); lcd3.print("Hi there");`
+     */
+    void couple();
+    void uncouple();
+
 private:
     /**
      * Sends the specified byte to the LCD via software SPI as data or a
@@ -275,6 +285,7 @@ class Nokia_LCD {
      */
     bool printCharacter(char character);
 
+    bool mCoupled = false;
     const uint8_t kClk_pin, kDin_pin, kDc_pin, kCe_pin, kRst_pin, kBl_pin;
     bool mInverted = false;
     const bool kUsingBacklight;