move "mage" to a separate module

Move the magefile related files to a submodule, so that it
does not become a dependency for logrus itself.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

ci/mage.go

@@ -0,0 +1,10 @@
+// +build ignore
+
+package main
+
+import (
+	"github.com/magefile/mage/mage"
+	"os"
+)
+
+func main() { os.Exit(mage.Main()) }