Color mapping not supported

[Maestrop82]

In the current version, simple color mapping in an scss file is not supported.

Example:

$shadow-black:              rgba(black, 0.3);
$dark-on-20:    #bcc2cc;
background-hover-primary:      rgba($dark-on-20, 0.10),

Error message for black mapping:

ERROR:qtsass.api:Failed to compile source code
Traceback (most recent call last):
  File "...\Anaconda3\Scripts\qtsass-script.py", line 9, in <module>
    sys.exit(main())
  File "...\Anaconda3\lib\site-packages\qtsass\cli.py", line 72, in main
    compile_filename(args.input, args.output)
  File "...\Anaconda3\lib\site-packages\qtsass\api.py", line 120, in compile_filename
    css = compile(string, **kwargs)
  File "...\Anaconda3\lib\site-packages\qtsass\api.py", line 94, in compile
    return qt_conform(sass.compile(**kwargs))
  File "...\Anaconda3\lib\site-packages\sass.py", line 738, in compile
    raise CompileError(v)
sass.CompileError: Error: Function rgba is missing argument $b.
        on line 46 of stdin
>> $shadow-black-secondary:    rgba(black, 0.7);

Error message for color variable mapping:

ERROR:qtsass.api:Failed to compile source code
Traceback (most recent call last):
  File "...\Anaconda3\Scripts\qtsass-script.py", line 9, in <module>
    sys.exit(main())
  File "...\Anaconda3\lib\site-packages\qtsass\cli.py", line 72, in main
    compile_filename(args.input, args.output)
  File "...\Anaconda3\lib\site-packages\qtsass\api.py", line 120, in compile_filename
    css = compile(string, **kwargs)
  File "...\Anaconda3\lib\site-packages\qtsass\api.py", line 94, in compile
    return qt_conform(sass.compile(**kwargs))
  File "...\Anaconda3\lib\site-packages\sass.py", line 738, in compile
    raise CompileError(v)
sass.CompileError: Error: Function rgba is missing argument $b.
        on line 162 of stdin
>> background-hover-primary:   rgba($dark-on-20, 0.10),

   ----------------------------^

The code currently only supports the 4argument color input here:

class SassColor(collections.namedtuple('SassColor', ('r', 'g', 'b', 'a'))):

    def __new__(cls, r, g, b, a):
        r = float(r)
        g = float(g)
        b = float(b)
        a = float(a)
        return super(SassColor, cls).__new__(cls, r, g, b, a)