feat(link): add external_link

[curbengh]

This feature is introduced due to perf cost of the external_link filter. When use in renderer, it can be applied directly to links, instead of a filter which has to first parse the whole site content to look for links.

To take advantage of the perf improvement, external_link: of the site config needs to be disabled.

Configuration:

external_link:
  enable: false

marked:
  external_link:
    enable: true

Some notes:

1. This feature is only applicable to []() markdown syntax, not raw <a>.
2. This is not a duplicate hexo's external_link as that works on <a> element, not markdown.
   • hexo's external_link applies to every <a> regardless the source is raw <a> or []() (that is later rendered as <a>)
   • this also means the plugin uses its own external_link config, separate from the external_link site config.
3. This feature applies to post and page, but not theme layout (see point 1).
4. This should be compatible with hexo's external_link, meaning both hexo and this renderer's external_link can be enabled at the same time. hexo's external_link does avoid duplicate attributes.
5. isExternalLink could not be used here as that utility uses site config (i.e. external_link site config), whereas this plugin has its own external_link config.

Ref: hexojs/hexo#3833 (comment)

[coveralls]

Coverage increased (+1.8%) to 95.313% when pulling b4e83a4 on curbengh:external-link into 2394623 on hexojs:master.

[curbengh]

nofollow could also be implemented, after this PR.