Support sublists and strikethrough in Markdown

Created by: zhil

I propose to add 2 improvements into markdown - sublists and strike tag shortcut. It would be great for the quick TODO lists

Redmine have it and its very suitable

* Admin
** todo 1
** todo 2
** -already implemented-
* -front-end-
** -done-
** -done2-
*** -level 3-
*** todo

redmine would convert it to

gitlab - to

Unfortunately, I dont have any ruby skills at all, I would appreciate if somebody will have time to get that code from https://github.com/redmine/redmine

Thanks in advance :)