A few fixes for the Windows console interface

-Made eraseLine actually erase the rest of the line instead of the line
 and all lines below
-Fully fixed the console-is-too-big-for-eraseScreen issue
-Added COMMON_LVB constants to windows.nim (originally defined in
 wincon.h), and substituted some corresponding hex values in terminal.nim
-Fixed setForegroundColor and setBackgroundColor so that the brightness
 of the text is also modified when going from bright colors to non-bright
 colors

STILL TO FIX: make sure that writeStyled behavior is the same as on
Linux shells