fix(ios): pause scanning with pausePreview method on iOS

While iOS can uniquely continue scanning without the preview being rendered, other platforms already

pause and resume scanning with the pausePreview and resumePreview methods, respectively. This makes

the iOS implementation match the others exactly.

Closes #12.

src/ios/QRScanner.swift

@@ -14,6 +14,7 @@ class QRScanner : CDVPlugin, AVCaptureMetadataOutputObjectsDelegate {
     var backCamera: AVCaptureDevice?
 
     var scanning: Bool = false
+    var paused: Bool = false;
     var nextScanningCommand: CDVInvokedUrlCommand?
 
     enum QRScannerError: Int32 {
@@ -258,11 +259,19 @@ class QRScanner : CDVPlugin, AVCaptureMetadataOutputObjectsDelegate {
     }
 
     func pausePreview(command: CDVInvokedUrlCommand) {
+        if(scanning){
+            paused = true;
+            scanning = false;
+        }
         captureVideoPreviewLayer?.connection.enabled = false
         self.getStatus(command)
     }
 
     func resumePreview(command: CDVInvokedUrlCommand) {
+        if(paused){
+            paused = false;
+            scanning = true;
+        }
         captureVideoPreviewLayer?.connection.enabled = true
         self.getStatus(command)
     }