title: NumberFormatter: allow "," and "." as the decimal separator
body: <p>I'm using <code>NumberFormatter</code> to format and validate user's currency input.
  Since some people are using <code>,</code> and the others <code>.</code> irrespective of the selected locale, I'd like to have both of those symbols treated as the decimal separator when processing an input (i.e. formatting a <code>String</code> to <code>Number</code>).</p>
  
  <p>How can I achieve this?</p>
  
  <p>My current <code>NumberFormatter</code> configuration is as follows:</p>
  
  <pre><code>    private lazy var currencyFormatter: NumberFormatter = {
          let currencyFormatter = NumberFormatter()
          currencyFormatter.numberStyle = .currency
          currencyFormatter.locale = Locale.autoupdatingCurrent
          currencyFormatter.currencySymbol = ""
          currencyFormatter.maximumFractionDigits = 2
          currencyFormatter.minimumFractionDigits = 2
          return currencyFormatter
      }()
  </code></pre>
  
  <p>Should I use multiple formatters to achieve the result I want?</p>
  
  <p><strong>The result</strong></p>
  
  <p>I'd like to have numbers <code>5,34</code> and <code>5.34</code> to be treated as valid and converted to the appropriate <code>NSNumber</code>. However, I'd like to still be able to reject <code>5.34lksdfns</code> as invalid.</p>