feat: add rss autodiscovery

[curbengh]

Many (most?) themes already offer this feature, but this plugin should've added it in the first place.

ref: http://www.rssboard.org/rss-autodiscovery
credit: meta_generator.js

[coveralls]

Coverage remained the same at 100.0% when pulling 77c95f9 on curbengh:autodiscovery into d66f225 on hexojs:master.

[SukkaW]

IMHO, the default config should be false.

[SukkaW]

README should be updated as well.

[curbengh]

IMHO, the default config should be false.

Please give a valid reason. It's part of the specification so most RSS reader program/plugin should support it and site with RSS is expected to have it. I would find it puzzling that users wouldn't want this feature turned on after installing this plugin.

[SukkaW]

@curbengh Many theme has already support RSS autodiscovery.

• NexT: https://github.com/theme-next/hexo-theme-next/blob/887bdf41019d7b1dee4f999b1d0072318090d95a/layout/_partials/head/head.swig#L29
• Material: https://github.com/viosey/hexo-theme-material/blob/d93c5a82db38d851dcb89961a42ef1ca09a9d677/layout/_partial/head.ejs#L149
• Icarus: https://github.com/ppoffice/hexo-theme-icarus/blob/8804d4612dd2b18ef8ca8734e9f5edde04e8af5e/layout/common/head.ejs#L25

RSS Autodiscovery is also one of the item from hexo-theme-unit-test Checklist
With this filter the generate performance might be worse.

[curbengh]

Many theme has already support RSS autodiscovery.

I'm aware of that, I already mentioned

#96 (comment)
Many (most?) themes already offer this feature, but this plugin should've added it in the first place.

---

RSS Autodiscovery is also one of the item from hexo-theme-unit-test Checklist

Popular themes indeed would (and should) pass that checklist, but we can't guarantee all available themes have that. Some themes disable by default as they didn't implement mechanism to add it automatically when hexo-generator-feed is installed.