feat(css_helper): add support for custom attributes

[curbengh]

What does it do?

This is mainly to add subresource integrity (SRI) to <layout>, though arbitrary attributes are supported as well.

Existing syntax is still supported (credit to #3681 (comment)).

Supported syntax:

• <%- css('layout.css') %>
• <%- css(['layout.css', 'gallery.css']) %>
• <%- css({href: 'layout.css', integrity: 'hash-value'}) %>
• <%- css({href: 'layout.css', integrity: 'hash-value'}) %>
• <%- css({href: 'layout.css', integrity: 'hash-value'}, {href: 'gallery.css', integrity: 'hash-stuff'}) %>
• <%- css([{href: 'layout.css', integrity: 'hash-value'}, {href: 'gallery.css', integrity: 'hash-stuff'}]) %>
• <%- css([{href: 'layout.css', integrity: 'hash-value'}, {href: 'gallery.css', integrity: 'hash-stuff'}]) %>

Related to #3681, without support of async nor defer (not used in <link>).

How to test

git clone -b sri-css https://github.com/curbengh/hexo.git
cd hexo
npm install
npm test

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage decreased (-0.01%) to 97.253% when pulling 814b4d8 on curbengh:sri-css into 0670d54 on hexojs:master.

[curbengh]

Ready for review.