Added an If condition for QRCode.Builder

[vicvoh]

This can be used for conditionally applying modifiers for QRCode.Builder

let qrcodebuilder = try! QRCode.build
   .text("Sample QR-code")
   .if(condition: true, configure: { builder in
      builder.background.cornerRadius(0.75)
   })

[dagronf]

I've just made some small changes to your request now that its merged :-

1. I removed the label requirement from the 'if' implementation, so that the if condition looks more 'natural'

 let qrcodebuilder = try QRCode.build
    .text("Sample QR-code")
    .if(<some_condition>) { builder in
        builder.background.cornerRadius(0.75)
    }

2. Added a 'discardableResult' to the function definition to be in line with the other builder functions
3. Added some basic tests to make sure the condition is handled correctly

Thank you so much for your work!