URLs in a['href'] should be escaped for LaTeX

An URL such as in a `a['href']` can contain URL escape codes, which all start with a `%`. The LaTeX-converter should escape all `%` to `\%` using `escape()`, or otherwise the unescaped `%` will be treated like a comment in LaTeX, corrupting the file.

This pull request fixes this by calling `escape()` on the URL.