Consider supporting jsconfig.json in compilation

[zhengbli]

We are adding support for using jsconfig.json files to define a project and compilation options in the language service, and we should consider supporting jsconfig.json files in tsc compilation as well.

Pros:

• In the cases of mixed .ts files and .js files, the user always needs only one json files for editing and compiling
• More compatible with existing jsconfig.json users and one less step for them to adopt TypeScript in their projects

Cons:

• It seems to be counter-intuitive behavior that a jsconfig drives the TypeScript compiler
• jsconfig.json and tsconfig.json have different default values for certain compiler options. For example, allowJs is default to true for jsconfig. This may lead to unexpected outcome.