envexp: add top-level Expand() using Getenv

Signed-off-by: Alejandro Mery <amery@jpi.io>

envexp/global.go

@@ -0,0 +1,14 @@
+package envexp
+
+import (
+	"os"
+)
+
+var envExpander = &expander{
+	get: os.Getenv,
+}
+
+// Expand uses os.GetEnv() to expand a given string
+func Expand(s string) string {
+	return envExpander.Expand(s)
+}