added color palette selector (customize palette)

[wckdouglas]

I implemented new buttons to pick color and add to a color list, undo the pick and clear the color list. The color list will be shown as palette_plot on a new tab and a R code line will be generated for simple copy-and-paste purpose.

[codecov-io]

Codecov Report

Merging #3 into master will decrease coverage by 1.57%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
- Coverage   18.03%   16.45%   -1.58%     
==========================================
  Files           9        9              
  Lines         438      480      +42     
==========================================
  Hits           79       79              
- Misses        359      401      +42

Impacted Files	Coverage Δ
R/HCL_color_picker.R	0% <0%> (ø)	:arrow_up:

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[clauswilke]

One more thing, more an idea than a requirement for merging at this time: It would be great if one could click on the colors in the palette plot to set that color in the color picker. With that, though, the unpick function would have to remove the currently selected color, not the last color that was picked.

[wckdouglas]

So turns out it is not too hard to implement targeted-color-removal. A minor problem now is that it is silently ignoring duplicated color selections. Ideally, a error/warning message seems to make more sense.

[clauswilke]

I made some additional improvements, including making the palette colors clickable. It's quite nice now I think.

[wckdouglas]

This does look much better!

Found a bug on allowing empty color selection, and is fixed in my new pull request