Add InitialShouldRemember Property to LoginDialogSettings

[shaggygi]

Is your feature request related to a problem? Please describe.
Add new bool property for LoginDialogSettings to initially check the RememberCheckBox. There are times when a user logins and wants to remember, so they check the box. When they are prompted for user/pass the next time, it would be nice to allow them to not have to check the box to remember each time. We need a new property to pass when calling ShowLoginAsync method.

Describe the solution you'd like
Be able to set the property when instantiating the LoginDialogSettings object like below:

var settings = new MahApps.Metro.Controls.Dialogs.LoginDialogSettings()
{
    InitialShouldRemember = true,  // New property here.
    RememberCheckBoxVisibility = Visibility.Visible
};

 LoginDialogData loginDialogData = 
                await (System.Windows.Application.Current.MainWindow as MetroWindow).ShowLoginAsync(
                    "Caption",
                    "Message",
                    settings);