feed_tag() helper

[SukkaW]

The idea of bring up a hexo built-in RSS autodiscovery helper comes from #115. I notice that there is already a feed_tag() helper: https://github.com/hexojs/hexo/blob/master/lib/plugins/helper/feed_tag.js

The problem is:

• The hexo's built-in feed_tag() doesn't support type
• The helper still requires manually setup.

We should bring up a helper that output autodiscovery tag automatically based on the config. Since it will be a helper maintained by us, it only requires theme developer to put something like <%- feed_autodiscovery() %> in the layout without worrying about the Breaking Changes of the plugins.

cc @AlynxZhou @stevenjoezhang

[SukkaW]

Fixed in hexojs/hexo#4029