title: Prevent input change from causing form to dirty (Angular 5.2)
body: <p>Is there a simple way to prevent an input control from dirtying its parent form?
  I could do so by building a custom tag that doesn't have a controlValueAccessor, but I was wondering if there is an easier way.</p>
  
  <p>In the middle of one of my forms I have an input that takes a number next to a button. The input is just to specify how many of a certain type of item I want to add to a set of things. The input where you specify how many is not relevant to whether the form should be considered dirty as there are no "changes" to the form until the add button is clicked at which point a bunch of items get added to the set and I propagate changes from there.</p>