Implement "prepare" mode: Let the user see and work with the calculated values (final paths, names, versions, etc) without having to build and inspect the results

[Black-Platypus]

Add "prepare" mode, calculating all major values and returning it to user, without having to "build" anything.
Use output object to build if options.mode is 'build'
This will allow for builder apps to better handle building apps using nw-builder without having to know in advance what the nwjs app is going to provide.
Using things like, for example,

var meta = await nwbuild({
	mode: 'prepare'
	// ...
});
if( !meta.app.legalCopyright && meta.app.company){
	meta.app.legalCopyright = "© " + meta.app.company + " " + ((new Date()).getFullYear());
}
// ...
// Now that we've done our changes, build using the previously arrived-at values
meta.mode = 'build';
await nwbuild(meta);

, a lot of things can be streamlined, without having to parse the app's manifest itself, without knowing in advance how nw-builder will parse/assign values, without having to make sure that any path resolves to what the builder app thinks/hopes it does (hopefully it should just be resolving things within the CWD, but who knows), etc

ToDo: Implement OSX behavior (has its own file, I'm not sure of the reasons, nor can I test the behavior)

Fixes: #1264

[ayushmanchhabra]

I had thought of something similar to this a while back but dropped the idea because I wanted to keep things as open as possible. Will review this last since this is a feature request.