Fix problems that commonmark has.


## Links

Links can't contain links.

```````````````````````````````` example
[<https://example.com>](/foo)
.
<p><a href="/foo">&lt;https://example.com&gt;</a></p>
````````````````````````````````

```````````````````````````````` example
[<name@example.com>](/foo)
.
<p><a href="/foo">&lt;name@example.com&gt;</a></p>
````````````````````````````````

## Emphasis

`<em>` doesn't contain `<em>`, `<strong>` doesn't contain `<strong>`.

```````````````````````````````` example
*_em_* __**strong**__ ______m______
.
<p><em>_em_</em> <strong>**strong**</strong> ______m______</p>
````````````````````````````````

### Non aggressive emphasis

```````````````````````````````` example
*a **b c* d**
.
<p><em>a **b c</em> d**</p>
````````````````````````````````

While CommonMark would render it into:

```
<p><em>a <em><em>b c</em> d</em></em></p>
```

```````````````````````````````` example
*a **b c* d**
.
<p><em>a **b c</em> d**</p>
````````````````````````````````

While CommonMark would render it into:

```
<p><em>a <em><em>b c</em> d</em></em></p>
```

What if the string is:

```````````````````````````````` example
*a **b c* d
.
<p><em>a **b c</em> d</p>
````````````````````````````````

CommonMark would still be a mess:

```
<p>*a *<em>b c</em> d</p>
```


## Max depth


```````````````````````````````` example
> > > > > > > > b
.
<blockquote>
<blockquote>
<blockquote>
<blockquote>
<blockquote>
<blockquote>
<p>&gt; &gt; b</p>
</blockquote>
</blockquote>
</blockquote>
</blockquote>
</blockquote>
</blockquote>
````````````````````````````````


```````````````````````````````` example
- a
  - b
    - c
      - d
        - e
          - f
            - g
              - h
.
<ul>
<li>a<ul>
<li>b<ul>
<li>c<ul>
<li>d<ul>
<li>e<ul>
<li>f
- g
- h</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
````````````````````````````````