Plugins - support a 'willSave' hook

Background angelozerr/tslint-language-service#32.

The tslint language server extension should support an option to auto fix lint rules with a fixer before a file is saved. The plugin API should support a willSave method that a plugin can use to perform an operation like auto fixing before the document is saved. In addition there needs to be request that the TypeScript language client like VS Code can use to inform the TypeScript language server that the user is about to save a file.

首先跟踪 issue 中描述的 plugin API 和 TypeScript language server 请求处理流程，然后查看 VS Code 这样的 TypeScript language client 在保存前如何进行通信。完成的标准是 plugins 可以实现 willSave 操作，并且 clients 可以在文件保存前通知 server。