Implement plain text starting with < in Diet templates.

This is the Jade way to emit conditional comments but works
with all plain text starting with <, too.

See http://jade-lang.com/reference/comments/.

source/vibe/templ/diet.d

@@ -581,6 +581,9 @@ private struct DietCompiler(TRANSLATE...)
 				output.pushNode("-}");
 			} else if( ln[0] == '|' ){ // plain text node
 				buildTextNodeWriter(output, ln[1 .. ln.length], level, prepend_whitespaces);
+			} else if( ln[0] == '<' ){ // plain text node starting with <
+				assertp(next_indent_level <= level, "Child elements for plain text starting with '<' are not supported.");
+				buildTextNodeWriter(output, ln, level, prepend_whitespaces);
 			} else if( ln[0] == ':' ){ // filter node (filtered raw text)
 				// find all child lines
 				size_t next_tag = m_lineIndex+1;