Add INPUT_PULLDOWN to keywords list.

Closes arduino#3534

Author: Chris--A

build/shared/lib/keywords.txt

@@ -9,6 +9,7 @@ HIGH	LITERAL2	Constants	RESERVED_WORD_2
 LOW	LITERAL2	Constants	RESERVED_WORD_2
 INPUT	LITERAL2	Constants	RESERVED_WORD_2
 INPUT_PULLUP	LITERAL2	Constants	RESERVED_WORD_2
+INPUT_PULLDOWN	LITERAL2	Constants	RESERVED_WORD_2
 OUTPUT	LITERAL2	Constants	RESERVED_WORD_2
 DEC	LITERAL2	Serial_Print	RESERVED_WORD_2
 BIN	LITERAL2	Serial_Print	RESERVED_WORD_2