PhoneInputFormatter: Allow '+' to be entered

[vHanda]

Hi. I'd love it if when I press the "+" button, a "+" would appear, instead of nothing. It took me sometime to figure out that the + would be added automatically.

Please let me know if this is a behavior you'd accept, and I'll see if/when I can find the time to contribute.

[caseyryan]

Hi! Yes, you're right. I also had this request from other people but I still had no time to implement it, unfortunately. Too loaded with work :(

[vHanda]

Thank you. You're amazing.

[caseyryan]

I have't pushed the project to pub.dev yet, because there's another bug fix in progress