Feature: Light mode

[applejag]

Would be nice to have a light mode when generating the image.

Use case is that our CLI program has both a dark and light mode, and we would like to preview both using termshot.

Mockup of how I envision it (super simple):

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Edit: Sorry, just realized that this is a potential duplicate of #80

[lukasz-lobocki]

You may try termos. It offers color controlling flags. But no shadow.