Add `background` option for disabling theme background colours

[OpenSauce04]

This pull request adds a new option which allows the user to configure whether the background colour of the theme is used.

This is useful for allowing users to have a transparent background with a simple configuration option, rather than manually editing any theme they use, which is currently the only alternative and is very unfriendly to the user.

I am unfamiliar with both Go and this codebase, so if I have made any mistakes in this pull request such as not following best practice or impropper code placement, please correct me.

Please also let me know if this is the correct way of addressing this issue or if you would rather a different approach is taken.

[usfbih8u]

Would be nice having something like this but per Buffer and configuring a color like it is done in colorschemes, so we could customize buffer backgrounds from plugins.