set a default version value for mig package Version variable

version.go

@@ -6,4 +6,16 @@
 
 package mig /* import "mig.ninja/mig" */
 
-var Version string = "20160429-0.51f628c"
+// Version can be set at compile time to indicate the version of MIG
+// components. You'd typically want to set this during install using flags
+// such as -ldflags "-X mig.ninja/mig.Version=20170913-0.06824ce0.dev" when
+// calling the go build tools.
+var Version string = ""
+
+func init() {
+	// If the default value of Version is not being specified using the build
+	// tools, just set a generic version identifier.
+	if Version == "" {
+		Version = "0.unversioned"
+	}
+}