feat: support output in both atom and rss2

[curbengh]

Fix #59

How to test:

package.json
-  "hexo-generator-feed": "^2.0.0"
+  "hexo-generator-feed": "curbengh/hexo-generator-feed#atom-rss2"

$ rm -rf node_modules/ && npm i

_config.yml
feed:
  type:
    - 'atom'
    - 'rss2'

$ hexo clean && hexo g

cc @mythsman

[curbengh]

WIP, missing doc and unit test.

[coveralls]

Coverage remained the same at 100.0% when pulling 7f41f0f on curbengh:atom-rss2 into d168026 on hexojs:master.

[curbengh]

Note that generator.js still output one file only, which is why it's called twice in index.js. This limitation also applies to the unit test, hence it doesn't check both atom and rss2 are generated; at least the test can check the order is correct.

The limitation doesn't apply to autodiscovery, so there is a unit test to check both values exist.

---

Although the code may be simpler if use type: "both" config, I prefer to explicitly use type: ['atom, 'rss2'] for config readability.

[curbengh]

Ready for review.

[curbengh]

Rebased. Ready for review.

[tomap]

Maybe add in the doc an example of yaml with both output & both path so people don't struggle to type it

[curbengh]

Maybe add in the doc an example of yaml with both output & both path

+1. Added.