Make sure relative URLs are preserved

Add tests to make sure we don't break relative URLs again.
Extracted common html flags and common extensions for easy access from
tests.

Closes issue #104, which was fixed as a side effect of cf6bfc9.
I think it would be cleaner to define these as two consts. E.g.,

```Go
const htmlFlags = 0 |
	HTML_USE_XHTML |
	HTML_USE_SMARTYPANTS |
	HTML_SMARTYPANTS_FRACTIONS |
	HTML_SMARTYPANTS_LATEX_DASHES
```