重复的代码高亮处理

[Qusic]

https://github.com/hexojs/hexo-renderer-marked/blob/master/lib/renderer.js#L41-L47
https://github.com/hexojs/hexo/blob/master/lib/plugins/filter/before_post_render/backtick_code_block.js

这里我觉得可以把 backtick_code_block 这个 filter 去掉。
代码高亮本来作为 renderer 的任务就很合适，其次，post 不一定是 markdown 格式，但是 before_post_render 里目前对于代码块的处理完全是针对 markdown 的，其它模版可能会有不同的格式。
另外，高亮配置也要在不同的地方都读取和判断显得比较不优雅。hexojs/hexo-renderer-marked#26

[stale]

This issue has been automatically marked as stale because lack of recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because lack of recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stevenjoezhang]

Fixed in hexojs/hexo-renderer-marked#134