feat: Add maxFilesSize option

[Flyrell]

Thanks for a great library @bleenco.
I'm working on file uploader that accepts multiple files. What I need is to set the maximum size of all the files user has selected. As far as I know, the following behaviour should do the trick without any breaking changes:

• default size would be Number.POSITIVE_INFINITY
• if the summed files size is exceeded, reject event should be fired and no files should be added

What do you think about it? I would probably be able to create a PR for it.