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).

首先检查现有的 tsconfig.json 配置选项，以及 issue 中链接的入口点 compiler/sys.ts。在实现之前，明确提议的自定义模块解析 API 及其范围。完成标准是：TypeScript 能够通过约定的配置钩子，对使用异构或自定义模块加载器的项目进行类型检查。