Add go.mod file to fix build with go 1.16

From the go 1.16 release notes:

	Module-aware mode is enabled by default, regardless of whether a
	go.mod file is present in the current working directory or a
	parent directory

My current understanding is that the vendor/ directory should still be
picked up by default.

go.mod

@@ -0,0 +1,3 @@
+module github.com/nmeum/hii
+
+go 1.16