Fix #113: Impossible to turn off validation error alert

[dvuckovic]

Issue #113 stems from a faulty assignment in the plugin installer:

    // show alert or not?
    formDI.validationErrorShowAlert = properties.validationErrorShowAlert || true

This will always default to true value in case properties.validationErrorShowAlert is "falsy".

We can fix this by checking if the option was defined first, and assign the default only if it wasn't.

     // show alert or not?
-    formDI.validationErrorShowAlert = properties.validationErrorShowAlert || true
+    formDI.validationErrorShowAlert = typeof properties.validationErrorShowAlert !== 'undefined'
+        ? Boolean(properties.validationErrorShowAlert)
+        : true