* Noting that (as of 0.14) zola does not pass much of a context when
rendering the 404 page, so things like current URL/path aren't
available
* To work around this the `menu` macro has been split into `menu_for`
which accepts a parameter for which item is active.
* The previous macro continues to work as is, forwarding the results of
figuring out which item is active to the new `menu_for` macro
* The added 404 page will then use the `menu_for` macro and pass in a
blank item
