Refactor RSS template

[Lruihao]

• zhangxinxu feed
• RSS intro

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• [ ] hugo-fixit/hugo-atom-feed#1
• [x] add params.feed

# Feed config for RSS, Atom and JSON Feed.
[params.feed]
  limit = -1
  # Front matter support ( .Param "feed').fullText
  fullText = false

• [x] Front matter change: hiddenFromRss => hiddenFromFeed
• [x] featuredimage in RSS and JSON feed
• [x] author name format compatibility (hugo embedded rss.xml#L2-L31)