Add `ls_colors` option to theme

[zwpaper]

As of now, we support theme for colors, and still use ls_colors for file name colors, how about adding the ls_colors option to the theme configuration file, and so that we can configure all the colors in one place.

also, there is something that needs to be covered:

1. the ls_colors in the theme should be optional, and fall back to LS_COLORS env
2. the one in themes should overwrite the env one

@meain

[meain]

That sounds good. LS_COLORS makes sense as an env since it is a common standard and multiple programs can make use of it, but having an additional option to configure it via the config file is not a bad idea.

[meain]

On second thought I was wondering what would be the use of being able to specify LS_COLORS specifically for lsd?