Missing FontSize Property for ProgressDialog/ShowProgressAsync

## What steps will reproduce this issue?

Unable to set FontSize for ProgressDialog.
### Expected outcome

Being able to set FontSize either using ShowProgressAsync parameter or using MetroDialogSettings.
## 
### Environment
- MahApps.Metro **v1.2.4**
- Windows **10**
- Visual Studio **2015**
- .NET Framework **4.5**
