Add arguments and path, clean up

[Cretezy]

Added {} argument, with path. Also cleaned up some parts of the code (make final when can be) and the README (also added undocumented features such as . strings).

[Skyost]

@Cretezy Okay so a few points :

1. First, thanks a lot for contributing !
2. You're totally right about the nested strings part.
3. The second commit is also good but I prefer a strong typing in the project (and Dart allows us to do so).
4. Why using a path argument with a getPathFunction ? It seems a bit duplicate to me as the default getPathFunction is exactly $path/language.json.
5. One last thing : you've added three transitive dependencies in example/pubspec.lock : petitparser, image and xml. Are they needed ?
6. I don't see the matter of putting final everywhere. There was a myth at the time of Java where everybody was saying that final was faster. It's clearly not the case in Dart 😄
7. I think named arguments is a better and cleaner option to add arguments.

[Skyost]

I've made the following changes : 1e49dbd, is it fair to you ?

[Cretezy]

For 3, final does do strong typing since you can't redeclare the type.

For 4, it's usually easier to give a path than a function, so support for both is nicer.

For 5, pub did that on its own. Don't know why.

For 6, it's usually a best practice since it can lead to fewer bugs from accidentally assigning something.

(on mobile right now, will check again later)

[Skyost]

Yeah I understand your point of view. But anyway, I prefer not to make everything final and to replace it with the class name. That's a convention I've been using for a while now.

The path still seems duplicate to me. Maybe creating a function that allows to create a getPathFunction directly from a path ?