The aim is to replace every \n in a <pre> with a <br /> because st
doesn't support pre.
However, ST doesn't support <br/> for some reason, only <br> or <br />.
We use to add some <br>s, but BeautifulSoup automatically adds a <br/>
when it sees a <br> (close the tag), which causes the clipping of the
rest of the file by ST. But if we replace every \n with a <br />,
BeautifulSoup automatically replaces it with <br/> (= ST bug)
So, we do exactly that, except that at the very end, when markdown2html
returns, we replace every <br/> with a <br />
We have to make sure that everything is converted to base64 (and
replaced in the src attribute) because otherwise ST3 messes up the
height of the images
