Added multiplier for darken / lighten amount

[LukasPietzschmann]

I like that glance is able to generate a nicely looking color scheme for its popup. But I thought it would be nice if the generated colors could be darkened or brightened even more / less.

Therefore, I added a new option in the config that controls this. I'm way too dumb to understand the "color science" behind the generation, so there might be a better way to integrate this feature than just multiplying the amount 😅