use realloc() when increasing the size of _params. addParameter() now returns bool

indicating the status of adding the parameter where false indicates it failed to
increase the size of _params and did not add the parameter.

WiFiManager.h

@@ -100,8 +100,8 @@ class WiFiManager
     void          setAPCallback( void (*func)(WiFiManager*) );
     //called when settings have been changed and connection was successful
     void          setSaveConfigCallback( void (*func)(void) );
-    //adds a custom parameter
-    void          addParameter(WiFiManagerParameter *p);
+    //adds a custom parameter, returns false on failure
+    bool          addParameter(WiFiManagerParameter *p);
     //if this is set, it will exit after config, even if connection is unsuccessful.
     void          setBreakAfterConfig(boolean shouldBreak);
     //if this is set, try WPS setup when starting (this will delay config portal for up to 2 mins)