Add support for writing custom color palettes

Created originally on Bitbucket by Hannes Breul

Fixes a longstanding issue with xlsx files generated in Apple Numbers: When opening an existing file that has an internal color palette (`indexedColors`), the original color palette is dropped from the file and gets reset to the default, causing the colors to be off. With these changes, the color palette will be saved to the new file.

See also: https://stackoverflow.com/questions/48883829/when-i-save-a-openpyxl-it-changes-the-colour-of-all-the-rows-columns-text