Add filter event for backtick code block

[leesei]

Provides a easier way to extend language supported:
hexojs/hexo-util#13

Tag plugins can also handle the event so user can use both backtick and tag.
Current implementations relied on before_post_render filter and internally look up backtick blocks, which is inefficient:
https://github.com/bubkoo/hexo-filter-fenced-code
https://github.com/akfish/hexo-math

Allows external highlighter:
#1300 and a whole bunch of highlight issues
We can move Highlight.js code and css to hexojs/hexo-highlight-js plugin.