The proposal of using monorepo

**Why should we use monorepo?**

For example, Hexo is dropping Node.js 8 at this moment. That means every CI config under every repo has to be updated. It is painful.

If we are using monorepo, we could write only one ci config for all packages (run eslint & all unit test cases at once).

Also, with monorepo we could maintain a global `eslintrc` / `mocharc` for all packages.

**What should be included in monorepos**

Only for those repos that are not hexo plugins, which means:

- repos like `hexo-util`, `hexo-i18n`, `hexo-pagination`, `hexo-server`, `hexo-cli`, etc. should be merged into monorepos.
- repos like `hexo-renderer-marked`, `hexo-filter-nofollow`, `hexo-uglify`, etc. should remain separated.

**When should we start**

Right after Hexo 5.0.0 released.

**What monorepo manager should we use?**

lerna.

**Any play ground?**

https://github.com/hexojs/hexo-monorepo

@hexojs/core and owners of @hexojs should have access to the repo.