feat: support 'disableNunjucks' in front-matter

[curbengh]

What does it do?

Requested by #3259 cc @noraj

Although the behavior of `{{ }}` (with backtick wrap) is now working as expected since v5, user may want to use {{ }} without backtick wrap for certain articles (post/page). With this PR, when disableNunjucks are disabled globally (default) or through a renderer, user can enable it per article; at the same time, it is also possible to disable it per article even when it is enabled globally.

enabling disableNunjucks will disable processing of {{ }} / {% %} and tag plugins.

How to test

git clone -b fm-disable-nunjucks https://github.com/curbengh/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

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

[coveralls]

Coverage increased (+0.0005%) to 98.3% when pulling c6b2691 on curbengh:fm-disable-nunjucks into e30605d on hexojs:master.

[SukkaW]

Could be done before checking the renderer.

[curbengh]

front-matter should override renderer in this case.