Fix/multiple concurrent file uploads

[paulblyth]

Hey @abramenal - hoping this is an acceptable changeset. I know there's an open PR (#227) but that looks like it has gone stale and looks like it might have had some issues.

Closes #203
Closes #234

There's a little bit of ES6 in here and I've not contributed to a Cypress plugin before so let me know if that's bad practice - and please be extra critical :)

Note: I also had to commit changes in the recipes using --no-verify due to some eslint errors, I didn't want to dig too much into those

[paulblyth]

@tommy-anderson I saw your comment - would you be able to give these changes a test drive in your migration?

Edit: You should be able to test by npm i git://github.com/paulblyth/cypress-file-upload.git#build/fix/multiple-file-uploads (it's the same branch just with the dist folder committed so you can install without having to worry about building)

[abramenal]

I've made small changes and tested it all locally. Great job @paulblyth, thank you!
It is also good that you mentioned issues with linting. Fixed all of them (I think), will work further on making the contribution process smooth.
And welcome to the contributors list ❤️

Next steps:

• merge to master
• wait for CI to be green (it will check the logic against some browsers in headed and headless modes)
• release it tomorrow, boom! 💥

[abramenal]

@all-contributors add @paulblyth for code

[allcontributors]

@abramenal

I've put up a pull request to add @paulblyth! 🎉

[paulblyth]

@abramenal thanks for cleaning up and merging 🙌

[abramenal]

Released in v5 btw