Parsing '[ (]([ (]([ (]([ (](...' takes quadratic time

[mity]

$ python -c 'print("[ (](" * 20000)' | time md2html > /dev/null
0.75user 0.00system 0:00.76elapsed 97%CPU (0avgtext+0avgdata 3440maxresident)k
0inputs+0outputs (0major+550minor)pagefaults 0swaps
$ python -c 'print("[ (](" * 40000)' | time md2html > /dev/null
2.96user 0.00system 0:03.00elapsed 98%CPU (0avgtext+0avgdata 4956maxresident)k
0inputs+0outputs (0major+989minor)pagefaults 0swaps
$ python -c 'print("[ (](" * 80000)' | time md2html > /dev/null
12.22user 0.00system 0:12.28elapsed 99%CPU (0avgtext+0avgdata 8568maxresident)k
0inputs+0outputs (0major+1867minor)pagefaults 0swaps

(the 3rd case from #57)