Add flutter-args variable

[storvik]

I was going to write an Emacs package for interacting with Flutter. Luckily I googled it and stumbled upon your package. Awesome package, very useful when working with Flutter. I have set after-save-hook to flutter-run-or-hot-reload which makes it hot reload on save, and everything is working smoothly.

The only thing I missed was a way to specify default flags to pass to Flutter. I am working with several projects, some web and some linux. When running flutter I had to C-u M-x flutter-run in order to add -d linux. With this commit it can be specified in .dir-locals.el and therefore set on a per-project basis. I've added a note about this to the readme.

This commit should not break existing functionality as flutter-args is set to nil by default, and then not used in flutter-run.

What do you think? Is this something you could consider adding?

Again, thanks for this package !

[amake]

Thanks for your contribution. I think the idea is a good one. I had some suggestions for improvements.

[storvik]

Thanks for the feedback @amake! Clever to use or instead, didn't think of that.