Implement `--keep-holes-filled-with-bg-color` option

[phseiff]

I should implement an option to fit the following description text (already added in the README, but commented out since it's not implemented yet:

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• --keep-holes-filled-with-bg-color <true or false>

  If set to true (the default is false), this option ensures that "holes" within emojis that are filled with the background color (as specified by --background) keep this background color rather than become transparent.
  
  This can be useful for example if you have black line art on an even white background, and you want every part of the background to be transparent except for the parts surrounded by lines (the "inside" of the lineart).
  The example image further up the page, for example, would've profited greatly from this.

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Feel free to tell me if you think this feature is important, or if you'd personally like to have this feature; it helps me in prioritizing what to do when!