Implement default color sets for slices

[mahozad]

See #26.

Add some default color sets to use as slice colors.
Each color set must be available in two variants: single color for the slice or gradient for the slice.

Can we somehow use themes and styles for this?
We can have some default styles in our library that users can use.

<resource>
  <style name="ColorfulStyle">
    <item name="gap">4dp</item>
    ...
  </style>
</resource>

<ir.mahoza.android.piechart
  style="@style/ColorfulStyle"
... />

Possible options could be:

• Monochrome: One shade of color with different lightness or saturation
• Colorful 1: bright colors
• Colorful 2: dim colors
• Colorful 3: ...
• Grayscale: varying shades of gray color