Make low fuel color configurable

src/c/settings.h

@@ -11,6 +11,7 @@ typedef struct ClaySettings {
   GColor ColorMGBars;
   GColor ColorMGTime;
   GColor ColorFG;
+  GColor ColorLowFuel;
 } __attribute__((__packed__)) ClaySettings;
 
 _Static_assert(sizeof(ClaySettings) <= 256, "ClaySettings exceeds Pebble 256-byte persist limit!");