refactor(highlight): add strip-indent

[SukkaW]

During writing proposal at hexojs/hexo#4010, I noticed strip-indent is used at hexo-renderer-marked and backtick_code filter.

Although strip-indent at hexo-renderer-marked will be removed in hexojs/hexo-renderer-marked#134, but after searching strip-indent at hexojs/hexo I found strip-indent mostly used for highlight in hexojs/hexo as well. It means we should add strip-indent inside highlightUtils.

After this PR is merged, strip-indent inside hexojs/hexo could be removed.

[coveralls]

Coverage increased (+0.005%) to 96.78% when pulling e077fa6 on SukkaW:add-strip-indent into c876271 on hexojs:master.