Add Vim modelines

Change-Id: I23f30cd9be56089498089f8b12abe3f3e186b525
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/argument.hpp b/argument.hpp
index 252fc36..3cb7e68 100644
--- a/argument.hpp
+++ b/argument.hpp
@@ -26,3 +26,5 @@
 };
 
 #endif
+
+// vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4