Add --watch mode

[ajafff]

This proposes to add a --watch option or a plugin module that does the actual watching.

• creates all the specified projects
  • regardless of whether it contains no files to lint -> these could be added later
• watch for changes in the TypeScript projects
  • watch failed lookup locations of initial tsconfig.json
• watch for changes in configuration files (requires configuration: return failed lookup locations #478)
• lint everything that is affected by the change (could be based on Caching lint results #370)
• report failures in all files, not just the ones affected by a change
• uses ts.createWatchProgram instead of ts.createProgram
• requires some major refactoring in runner.ts
• reusing SourceFiles is probably no longer possible if multiple Programs are watched at the same time
• TypeScript should be able to access environment variables, because file watching can be affected by that

TBD:

• how does this interact with --fix?
• Display some message when starting a new run due to changes?
• how to test this?
• requires a way to invalidate caches
• what to use for watching of raw glob patterns if --project is not specified?