allow setting colors via NEWT_COLORS or NEWT_COLORS_FILE variables
    
    Default color palette can be now modified by color specifications as
    "root=,black:button=red,yellow", the string can be set in NEWT_COLORS
    environment variable or in a file which is specified by NEWT_COLORS_FILE
    variable.
    
    Default NEWT_COLORS_FILE can be set by --with-colorsfile option.