Support wysiwyg newline

Hi @chjj

## Changes

1. fixed heading syntax error.
2. add options wysiwyg line break

### fixed heading syntax error.

This syntax is standard markdown syntax that is no support other markdown editor and github

```
// It is not heading syntax
[space]# heading
[space]#heading
blah blah blah #heading


//it heading
# heading
# heading
# blah blah blah blah blah blah #blah blah blah
```

### add options wysiwyg line break

wysiwyg is option for simply new line break like wysiwyg editor and  github issue.


if wysiwyg is true : 

```
Hi all.
This is insanehong


I'm currently at naver corp as a front-end engineer.
bye see you.
```

#### Github Support this syntax on issue follow: 

Hi all.
This is insanehong


I'm currently at naver corp as a front-end engineer.
bye see you.