Introduce nesting limit (#87)

* support fnDepthLimit

* fix parse option

* rename fnDepthLimit to nestLimit

* support limit nesting: big, bold, small, italic, strike

* improve mention

* fix hashtag

* support limit nesting: hashtag

* support limit nesting: url

* refine link label

* refactor

* fix link

* change default value of nestLimit

* fix link label

* add test

* restore and modify test