- introduced a `select_option` macro for building <option> elements
- introduced a `checkbox` macro for building an <input type="checkbox"> and related elements
- moved the `plural` macro in macros.html, and added the keepzero optional argument
- changed order of tests in ctxtnav for clarification of the code
- always place `title` attribute before `href` attribute, in <a> tags.
It makes it easier to quicly grasp what's the link role.
- shorten excessively long lines (now max ~120)
