data: allow running pipeline without wiping temp

Data pipeline ran with separate versions use different folders, so it
makes sense to allow running multiple data pipelines without wiping in
between.

The default is still to wipe clean, so nobody will get caught by
surprise.

tasks/data.js

@@ -4,7 +4,9 @@ module.exports = function (grunt) {
 	grunt.registerTask('data', 'Build timezone data.', function (version) {
 		version = version || 'latest';
 
-		grunt.task.run('clean:data');
+		if (!grunt.option('dirty-run')) {
+			grunt.task.run('clean:data');
+		}
 
 		grunt.task.run([
 			'data-download:' + version,