Set the background to "default" instead of BackgroundColor

[Jaykul]

As of Windows 10 build 18298, the new “Terminal” settings allow setting default ForegroundColor and BackgroundColor values which are separate from the 16 color "ConsoleColor" palette.

PowerLine doesn't use the ANSI/VT escape sequence for default as the default background, but instead specified $Host.UI.RawUI.BackgroundColor -- which no longer matches the actual background color if the user sets the terminal settings.