Mocha fix for Windows, `tsconfig.json` and `launch.json` simplified

[borekb]

I had trouble running this on Windows (#3) so experimented with the settings a bit. Summary of this PR:

• Mocha configuration that should work cross-platform (workaround for a possible VSCode bug https://github.com/Microsoft/vscode/issues/34541). Could someone test it please on a Mac / Linux?
• launch.json and tsconfig.json simplified to a bare minimum.
• README now contains a couple of screenshots 🖼