migrate: Add support CQL comment callbacks

This patch adds a new migration event type CallComment that it triggered by adding `-- CALL <name>;` comment in a CQL file.

Fixes #101

migrate/export_test.go

@@ -0,0 +1,5 @@
+package migrate
+
+func IsCallback(stmt string) (name string) {
+	return isCallback(stmt)
+}