Feature: If RSS is enabled, show RSS icon in menu

[vvzen]

This PR 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:

menu_items = [
    {name = "home", url = "$BASE_URL"},

    # [...]

    # RSS
    {name = "", url = "$BASE_URL/$FEED_FILENAME"},
]

It also requires that the user has a config.toml that contains the following entries, and generate_feed is set to true :

generate_feed = true
feed_filename = "rss.xml"
author = "Your name here"

The icon would look like this:

A live example can be seen here: https://valerioviperino.me