Fix for block quotes and question about different levels of block quotes

To fix generating block quotes, the [line 132](https://github.com/stamby/md-to-html/blob/a394eaef230a90dd5af91d03c453b0474a41358a/md-to-html.sed#L132C5-L132C50) should be changed to

```sed
s/(^|\n) *([^\n]+)/\1<p>\2<\/p>/g
```

And another question: are different levels of block quotes in lines 134-138 really needed?