DataGridNumericUpDownColumn StringFormat binding

## What steps will reproduce this issue?

StringFormat is not a dependencyProperty

### Expected outcome
Be able to use this on xaml:
StringFormat="{Binding SomeFormatNumber}"

### Environment

- MahApps.Metro: 1.5
- Windows OS: Windows 10
- Visual Studio: 2017
- .NET Framework: 4.5.2
