feat: custom template

[curbengh]

As suggested in #101 (comment), cc @freak3dot

How to test:

package.json
-  "hexo-generator-feed": "^2.0.0"
+  "hexo-generator-feed": "curbengh/hexo-generator-feed#custom-template"

$ rm -rf node_modules/ && npm i

_config.yml
# Optional, prevent custom.xml from being copied into public/ folder
exclude:
  - 'custom.xml'
feed:
  type: 'atom'
  template: './source/custom.xml'

$ hexo clean && hexo g

---

Following configs are supported:

feed:
  type:
    - 'atom'
    - 'rss2'
  template:
    - './source/_atom.xml'
    - './source/_rss2.xml'

feed:
  type:
    - 'atom'
    - 'rss2'
  template:
    - './source/_atom.xml'
  # rss2 uses the default template instead

[coveralls]

Coverage remained the same at 100.0% when pulling 2e54192 on curbengh:custom-template into 6e5bb89 on hexojs:master.

[SukkaW]

LGTM!