url-encode book section titles

Some of the translated section titles end in question marks.
When these are placed directly into a link without quoting,
the trailing "?" is parsed as a parameter separator, leading
to a broken link.

The same problem was fixed in #149, but only for the actual
section listing. This commit fixes the prev/next links, as
well as any direct chapter links.
Merge pull request #259 from github/urlencode-sections

url-encode book section titles