Expose configuration hook for custom module resolution

Following on from #6012 and #5039, it'd be nice to have the ability to add TypeScript to projects with heterogeneous module resolution/packaging systems and have typechecking work.

The paths and baseUrl configuration options get us a long way, but they don't deal well with module loaders that are less widely used, frequently changing, or that allow for complex module resolution rules. Letting the user entirely outsource all module resolution concerns using the configuration would allow type checking to work regardless of the user's preference of npm, bower, requirejs, browserify, jspm, webpack, etc.

A simple approach might be to have a tsconfig option for a parametrized command line invocation (similar to npm scripts) that is able to resolve modules. Each package manager can distribute a tool that implements module resolution according to its own needs, which the users can install and point to in their tsconfig.json.

Exactly how extensive the API needs to be is up for debate. It could be something as simple as a function that maps requested module names to file system paths, or a more abstract implementation of something like System from here, which could open up more interesting use cases (e.g. "virtual" modules built using code generation, or F# style type providers).

まず、既存の tsconfig.json の設定オプションと、issue でリンクされているエントリポイント compiler/sys.ts を確認します。実装前に、提案されているカスタムモジュール解決 API とそのスコープを明確にします。合意した設定フックを通じて、異種またはカスタムのモジュールローダーを使用するプロジェクトを TypeScript が型チェックできれば完了です。