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.

まず、issueで説明されているplugin APIとTypeScript language serverのリクエスト処理をたどり、その後、VS CodeのようなTypeScript language clientが保存前にどのように通信するかを確認します。pluginsがwillSave操作を実装でき、clientsがファイルの保存前にserverへ通知できれば完了です。