Add Eta support to Hexo

[bgub]

Hexo has support for EJS.

Since Eta is more powerful, faster, and has more reliable parsing than EJS, it would be great to add support for Eta to Hexo.

https://github.com/hexojs/hexo-renderer-ejs is installed by default in new Hexo projects. Since Eta imitates much of the external API of EJS, we should be able to use very similar code (the actual renderer is only 15 lines long). Then we should be able to add support for Eta-specific features, like plugins.