Roadmap for working release

[nathankot]

@terhechte as per #1, here's a list of things I think need to be done. Feel free to edit at will.

• [x] Clean up useless -project and -workspace flags, via #3
• [x] Ensure vanilla completion is working without project support, via #3
• [x] Ensure completion function is async (currently not), via #3
• [x] Annotations and type information when displaying candidates, via #3
• [x] Typing after completion candidates are built should instantly narrow down results, via #5
• [x] Add project support via --compilerargs, see #4
• [x] Write readme!
• [x] Keep XPC running? Might be nice if sourcekitten can run as daemon, see https://github.com/terhechte/SourceKittenDaemon

For project support we can simply check for common cocoapods/carthage framework paths and add them with -F. -sdk and -target detection might be more involving...