Return the filename from the current line.

def _compile(cls, lines):
        '''Return the filename from the current line.'''
        m = cls.RE_EXTEND.match(lines.current)
        if m is None:
            raise DefineBlockError('''Incorrect block definition at line {}, {}
Should be something like: #extend path/foo.html:'''.format(
                lines.pos, lines.current))
        return m.group(1)