Force positive sign

[LuisCardeBu]

I just can't find a way to force the positive sign prefix. Optometrists use negative and positive numbers in their prescriptions and I want to force the positive sign when numbers are positive.

[rxaviers]

Number formatter doesn't provide such option, but we can accept such enhancement if you're willing to implement it. In such case, I'd recommend discussing the API (option name and high level functionality) either in this issue (or in #437), then go for PR.

Thanks!

Cross links:

• https://www.unicode.org/reports/tr35/tr35-numbers.html#Explicit_Plus
• Plus sign on positive numbers #437 (comment)

Dup of #437