This adds support for showing a tiny RSS svg badge in the top menu in
case the user added a menu_item that looks like this:
`{name = "", url = "$BASE_URL/$FEED_FILENAME"}`
This also requires that the user has a config.toml that contains the
following entries:
generate_feed = true
feed_filename = "rss.xml"
author = "your name here"
* 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
