title: Allow upto 3 decimal points value in flutter InputTextType.number in flutter
body: <p>Below is the code I have tried but it completely fails. If I Remove the <code>WhitelistingTextInputFormatter</code>, I get the Number keyboard and I can insert numbers and periods. but the number of <code>period</code> I can use is more than one, which I need to limit at just one. how to do this?</p>
  
  <pre><code>            TextField(
                    controller: _weightCtr,
                    keyboardType: TextInputType.numberWithOptions(decimal: true),
                    inputFormatters: [
                      BlacklistingTextInputFormatter(new RegExp('[\\-|\\ ]')),
                      WhitelistingTextInputFormatter(new RegExp('^\d+[\.\,]\d+\$')),
                    ],
                    decoration: InputDecoration(
                      hintText: "Please enter a valid weight for this trip",
                    ),
                    style: Theme.of(context).textTheme.title.copyWith(
                      fontWeight: FontWeight.w300,
                      fontSize: 14,
                    ),
                  ),
  </code></pre>