perf: avoid running irrelevant plugins in 'clean' command

[curbengh]

What does it do?

Loading config and plugins are not needed in hexo clean. Noticed from #4381.
Only filter plugins are executed, i.e. to execute after_clean filter,

How to test

git clone -b BRANCH https://github.com/USER/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

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

[coveralls]

Coverage remained the same at 97.748% when pulling 33dcc90 on curbengh:console-clean into 65961d3 on hexojs:master.

[curbengh]

Initial draft used cmd.startsWith('c'), however in light of #4388, it must now match 'clean'.

#4388 (i.e. alias[this.env.cmd] === 'clean') could not be used here because console plugins (consequently their command aliases) have not been registered at this point.

This means using alias like hexo c cannot benefit from this shortcut. To retain hexo c alias (and avoiding breaking change), clean console plugin is still registered in "plugins/console.js".