Support all tags

Instead of allowing only a subset of tag names, `slugify` the tag name
so that the URL is always valid.

This is the same thing done on "jekyll-archives":
-
https://github.com/jekyll/jekyll-archives/blob/master/lib/jekyll-archives/archive.rb#L132

lib/jekyll-feed/generator.rb

@@ -84,12 +84,9 @@ def generate_feed_by_tag
 
     def generate_tag_feed(tags_pool, tags_path)
       tags_pool.each do |tag|
-        # allow only tags with basic alphanumeric characters and underscore to keep
-        # feed path simple.
-        next if %r![^a-zA-Z0-9_]!.match?(tag)
-
+        tag_slug = Jekyll::Utils.slugify(tag)
         Jekyll.logger.info "Jekyll Feed:", "Generating feed for posts tagged #{tag}"
-        path = "#{tags_path}#{tag}.xml"
+        path = "#{tags_path}#{tag_slug}.xml"
         next if file_exists?(path)
 
         @site.pages << make_page(path, :tags => tag)