Respect skim environment variables

[mk12]

Skim's default colors do not play well with my base16 terminal setup.

Skim has SKIM_DEFAULT_OPTIONS and SKIM_DEFAULT_COMMAND. I was hoping to just copy my fzf equivalents into those. However, only the skim binary reads those, not the library. There's an issue about it here: skim-rs/skim#390.

Once skim provides a helper to read those variables and create SkimOptions, I think git-branchless should use that. Or maybe in the meantime it could duplicate some of the logic to do that, or use its own environment variables to configure things like color.

[arxanas]

Thanks for the suggestion. I agree that it would be good to configure the Skim viewer. It's not a priority for me right now, but I'm happy to accept a PR to do this.