style: no-useless-concat & one-var (uninitialized)

[curbengh]

What does it do?

no-useless-concat somehow is not enforced in recent commits.

I also remove rules override in .eslintrc.json for consistency across hexo projects.

Removing rules override is also in preparation for prettier to avoid conflicting rules (e.g. [1], [2]).

How to test

git clone -b no-useless-concat 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 remained the same at 97.17% when pulling e690eca on curbengh:no-useless-concat into c1bbac3 on hexojs:master.

[yoshinorin]

Before we merge this PR, we should discuss use the prettier or not.

As you know active maintainers discussed this matter hexojs/eslint-config-hexo#7
But, it seems maybe not yet decided it.

PS: I agree to use prettier

[curbengh]

Even without prettier, for consistency, we shouldn't override eslint-config-hexo in the first place.

[curbengh]

Rebased

[curbengh]

Ready for review.

[curbengh]

@hexojs/core
ready for review.