Add <hr> support

old:
https://github.com/p01/mmd.js/blob/cd7f4872174bb29ef853f880119909dc70b7b4eb/mmd.js#L23-L25

new:
```js
.forEach(function(b, f, R) 
 {
        if (b === "----") {  // 👈 add this
             h += `<hr>`
             return
        } 
 	f=b[0]; 
        // ...
```