Add support for ARGB hex strings

[chrishoage]

Polybar only supports #ARGB for transparency. I previously used the workaround in #132 (comment), however this is not a particularly clean solution.

With this change I can simply import

~/.cache/wal/colors-polybar.ini and have the #ARGB pre-defined with no need to pass in the color via an environment variable

[chrishoage]

I didn't find any tests for the Color class, otherwise I would have updated those.

[chrishoage]

I'm unsure about this name. Happy to tweak it to your liking.

[chrishoage]

If this is not acceptable another alternative I would be open to is {background.alpha_hex} similar to {background.alpha_dec}

pywal/pywal/util.py

Lines 46 to 49 in 4997a49

	@property
	def alpha_dec(self):
	"""Export the alpha value as a decimal number in [0, 1]."""
	return int(self.alpha_num) / 100

That way I could achieve the same thing in my colors-polybar.ini using built-ins

[colors]
background=#{background.alpha_hex}{background.strip}

In fact, I think I will implement this in addition to what I have.

Please let me know if you would like me to remove hex_argb