Allow passing __FlashStringHelper* in addition to char* everywhere

Arduino supplies the F() macro that allows easily putting strings in
PROGMEM and marking them with a custom type. Essentially,
a __FlashStringHelper* is just the same char*, but cast to a different
type, which allows for example Serial.print() to automatically handle
both PROGMEM and normal strings.

By letting this library accept __FlashStringHelper* as well, you can
use easily use the F() macro and reduce the chance of mixing up normal
and PROGMEM pointers.

Adafruit_MQTT.cpp

@@ -83,6 +83,20 @@ Adafruit_MQTT::Adafruit_MQTT(const char *server, uint16_t port, const char *cid,
   }
 }
 
+Adafruit_MQTT::Adafruit_MQTT(const __FlashStringHelper *server, uint16_t port, const __FlashStringHelper *cid,
+                             const __FlashStringHelper *user, const __FlashStringHelper *pass) {
+  servername = (const char *)server;
+  portnum = port;
+  clientid = (const char *)cid;
+  username = (const char *)user;
+  password = (const char *)pass;
+
+  for (uint8_t i=0; i<MAXSUBSCRIPTIONS; i++) {
+    subscriptions[i] = 0;
+  }
+}
+
+
 /*
 Adafruit_MQTT::Adafruit_MQTT(char *server, uint16_t port, char *cid, char *user, char *pass) {
   strncpy(servername, server, SERVERNAME_SIZE);
@@ -372,6 +386,13 @@ Adafruit_MQTT_Publish::Adafruit_MQTT_Publish(Adafruit_MQTT *mqttserver,
   qos = q;
 }
 
+Adafruit_MQTT_Publish::Adafruit_MQTT_Publish(Adafruit_MQTT *mqttserver,
+                                             const __FlashStringHelper *feed, uint8_t q) {
+  mqtt = mqttserver;
+  topic = (const char *)feed;
+  qos = q;
+}
+
 bool Adafruit_MQTT_Publish::publish(int32_t i) {
   char payload[18];
   itoa(i, payload, 10);
@@ -403,3 +424,10 @@ Adafruit_MQTT_Subscribe::Adafruit_MQTT_Subscribe(Adafruit_MQTT *mqttserver,
   topic = feed;
   qos = q;
 }
+
+Adafruit_MQTT_Subscribe::Adafruit_MQTT_Subscribe(Adafruit_MQTT *mqttserver,
+                                                 const __FlashStringHelper *feed, uint8_t q) {
+  mqtt = mqttserver;
+  topic = (const char *)feed;
+  qos = q;
+}

Adafruit_MQTT.h

@@ -94,6 +94,8 @@ class Adafruit_MQTT {
  public:
   Adafruit_MQTT(const char *server, uint16_t port, const char *cid,
                 const char *user, const char *pass);
+  Adafruit_MQTT(const __FlashStringHelper *server, uint16_t port, const __FlashStringHelper *cid,
+                const __FlashStringHelper *user, const __FlashStringHelper *pass);
   virtual ~Adafruit_MQTT() {}
 
   // Connect to the MQTT server.  Returns 0 on success, otherwise an error code
@@ -124,7 +126,12 @@ class Adafruit_MQTT {
 
   // Publish a message to a topic using the specified QoS level.  Returns true
   // if the message was published, false otherwise.
+  // The topic must be stored in PROGMEM. It can either be a
+  // char*, or a __FlashStringHelper* (the result of the F() macro).
   bool publish(const char *topic, const char *payload, uint8_t qos);
+  bool publish(const __FlashStringHelper *topic, const char *payload, uint8_t qos) {
+    return publish((const char *)topic, payload, qos);
+  }
 
   // Add a subscription to receive messages for a topic.  Returns true if the
   // subscription could be added or was already present, false otherwise.
@@ -180,6 +187,7 @@ class Adafruit_MQTT {
 class Adafruit_MQTT_Publish {
  public:
   Adafruit_MQTT_Publish(Adafruit_MQTT *mqttserver, const char *feed, uint8_t qos = 0);
+  Adafruit_MQTT_Publish(Adafruit_MQTT *mqttserver, const __FlashStringHelper *feed, uint8_t qos = 0);
 
   bool publish(const char *s);
   bool publish(double f, uint8_t precision=2);  // Precision controls the minimum number of digits after decimal.
@@ -196,6 +204,7 @@ private:
 class Adafruit_MQTT_Subscribe {
  public:
   Adafruit_MQTT_Subscribe(Adafruit_MQTT *mqttserver, const char *feedname, uint8_t q=0);
+  Adafruit_MQTT_Subscribe(Adafruit_MQTT *mqttserver, const __FlashStringHelper *feedname, uint8_t q=0);
 
   bool setCallback(void (*callback)(char *));