mako placeholder

./mako/lexer.py:390:        match = self.match(r"\${")
./mako/parsetree.py:305:                for x in re.compile(r'(\${.+?})',
./mako/parsetree.py:307:                    m = re.compile(r'^\${(.+?)}$', re.S).match(x)
./mako/parsetree.py:324:                if re.search(r'\${.+?}', self.attributes[key]):