Add String::toDouble from upstream Arduino core (esp8266#5986)

Fixes esp8266#5985

Author: earlephilhower

cores/esp8266/WString.cpp

@@ -829,17 +829,24 @@ void String::trim(void) {
 // /*********************************************/
 
 long String::toInt(void) const {
-    if(buffer())
+    if (buffer())
         return atol(buffer());
     return 0;
 }
 
 float String::toFloat(void) const {
-    if(buffer())
+    if (buffer())
         return atof(buffer());
     return 0;
 }
 
+double String::toDouble(void) const
+{
+    if (buffer())
+        return atof(buffer());
+    return 0.0;
+}
+
 // global empty string to allow returning const String& with nothing
 
 const String emptyString;

cores/esp8266/WString.h

@@ -241,6 +241,7 @@ class String {
         // parsing/conversion
         long toInt(void) const;
         float toFloat(void) const;
+	double toDouble(void) const;
 
     protected:
         // Contains the string info when we're not in SSO mode