add optional custom colorpalettes to askcolor

[dende]

Hi, first of all thanks for the package! I'm using it to create a portable application that offers the same colorpicking UX on windows and on linux, because the windows colorpicker is not customizable.

In my example, I'm, using a colorpicker to select colors for plots that I draw with matplotlib. For these plots, I want to be using a palette of predefined colors that work good on charts. Such palettes could be found at ColorBrewer, for example.

With this commit you could easily use palettes like this:

# taken from https://colorbrewer2.org/#type=diverging&scheme=RdYlBu&n=11
custom_palette = ["#a50026", "#d73027", "#f46d43", "#fdae61", "#fee090", "#ffffbf", "#e0f3f8", "#abd9e9", "#74add1",
                  "#4575b4", "#313695"]

print(askcolor((255, 255, 0), root, palette=custom_palette))

[codecov[bot]]

Codecov Report

Merging #7 (f711c60) into master (407fe0e) will increase coverage by 0.16%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
+ Coverage   99.28%   99.44%   +0.16%     
==========================================
  Files           9        9              
  Lines        1265     1270       +5     
==========================================
+ Hits         1256     1263       +7     
+ Misses          9        7       -2     

Impacted Files	Coverage Δ
tkcolorpicker/colorpicker.py	98.52% <75.00%> (-0.48%)	:arrow_down:
tests.py	99.78% <0.00%> (+0.42%)	:arrow_up:
tkcolorpicker/functions.py	100.00% <0.00%> (+3.50%)	:arrow_up:

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