refactor: better usage of fragment_cache

[SukkaW]

After have a close look at hexojs/hexo#2164, I have realized something.

Although configuration file (both site's _config.yml and theme's _config.yml) is loaded only once, but the value has to be readed from parsed config (inside Locals) every time when meets in layout. So it is important to wrap config value inside fragment_cache.

I have ran through a benchmark with same setup I used in hexojs/hexo#3663. Before c8d5d16, is takes about 6.16 seconds to generate a site with 300 posts, After c8d5d16, it drops to about 5.9 seconds, which is about 4% less.

This PR is a PoC that we should wrap <%= config.xxx %> or <%= theme.xxx %> inside fragment_cache as much as possible.

[stevenjoezhang]

Is it ready to merge?

[SukkaW]

Fragment Cache is not the best way to do this. We need to use a lower-level of caching.