Add support for ARGB hex strings

[chrishoage]

Polybar only supports #ARGB for transparency. I previously used the workaround in https://github.com/dylanaraps/pywal/issues/132#issuecomment-352296339, 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]

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

https://github.com/dylanaraps/pywal/blob/4997a49eb7479ccda1228f064d6c50e1abedc5b4/pywal/util.py#L46-L49

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