[feature/template-engine] Allow leading underscores in variable names.

Subsilver uses ._file in overall_header.

PHPBB3-9726

tests/template/template_test.php

@@ -235,6 +235,15 @@ class phpbb_template_template_test extends phpbb_test_case
 				array('loop.inner'),
 				"first\n0\n0\n2\nx\nset\n1\n1\n2\ny\nset\nlast",
 			),*/
+			array(
+				// Just like a regular loop but the name begins
+				// with an underscore
+				'loop_underscore.html',
+				array(),
+				array(),
+				array(),
+				"noloop\nnoloop",
+			),
 			array(
 				'lang.html',
 				array(),