check the validity of heading

parsedown does not check the validity of the title heading

## examples

~~~
#######
~~~
#######

| GFM | parsedown |
|------- | -------------- |
| OK    | `<h6></h6>` |

~~~
############# 1 ##############
~~~
############## 1 #############

| GFM | parsedown | markdown extra |
|------- | -------------- | ------------------- |
| OK    | `<h27></h27>` | `<h6>######## 1</h6>` |