Support my plugin in fn plugin for RD style

RD style expands

    ((-((<20130714>))-))

to

    <%= fn <<-'0000random_id1111'
    <%=my "20130714","20130714"%>
    0000random_id1111
    %>

"%>" in "<%= ... %>" confuses ERB:

    % echo '<%= eval("<= 1 + 1 %>") %>' | erb -x
    #coding:ASCII-8BIT
    _erbout = ''; _erbout.concat(( eval("<= 1 + 1 ).to_s); _erbout.concat "\") %>\n"
    ; _erbout.force_encoding(__ENCODING__)

'eval("<%= 1 + 1 )...' is broken.

So "%>" should be escaped as "%%>".