Add a prompt / response system

[jason0x43]

Add functionality to Intern for asking the user for input. This function should be exported on the Intern object and be fairly basic, like:

const name = await intern.askUser('What is your name?');
const toTest = await intern.askUser('What files should be tested?', ['Specific file', 'All', 'Only changed']);

This functionality can be used for various purposes, such as Intern affecting watch mode behavior, a la Jest, or plugins allowing options to be set interactively.