Add helper functions for argument processing.

This is in preparation for the key & value API.
The functions in args_helper.h can be used without the need of
defining a usage_exit() function in the clients, while the functions
in args.h now are wrapper functions that behave just like before.

BUG=aomedia:2875

Change-Id: I76f174e1f3955721d9de03a4060854a12b922079
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0161749..cb7d5e6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -128,6 +128,8 @@
             "${AOM_ROOT}/aom/src/aom_integer.c")
 
 list(APPEND AOM_COMMON_APP_UTIL_SOURCES
+            "${AOM_ROOT}/common/args_helper.c"
+            "${AOM_ROOT}/common/args_helper.h"
             "${AOM_ROOT}/common/args.c"
             "${AOM_ROOT}/common/args.h"
             "${AOM_ROOT}/common/av1_config.c"