support more then 6 level

[zhuzhzh]

a

aa

aaa

aaaa

aaaaa

aaaaaa

####### aaaaaaa

the last line can't be showed in https://markmap.js.org/repl

[B1zguy]

I found a way around by tab-indenting each line and using hyphens/asterisks
instead.

Took me awhile to figure this out!