O(n^2) for * * * * .... a

[mity]

$ time python -c 'print("* " * 10000 + "a")' | ./target/release/comrak >/dev/null
real    0m1.054s
user    0m1.046s
sys     0m0.009s

$ time python -c 'print("* " * 20000 + "a")' | ./target/release/comrak >/dev/null
real    0m4.244s
user    0m4.207s
sys     0m0.037s

$ time python -c 'print("* " * 40000 + "a")' | ./target/release/comrak >/dev/null
real    0m20.608s
user    0m20.306s
sys     0m0.301s