Support for user-defined color theme

E.g. Vim colorschemes may rely on the hosting terminal's colors, thus it'd be handy to have Kings Cross loading the theme at runtime. I'm not sure where it should load from but these are the things that first come to my mind:

• Loading Xresources: it'd allow users to share the same config across different terminal emulators. Also it's easy for people who do dotfiles kind of thing
• Loading some custom config file (e.g. under XDG config dir): also good for dotfiles
• Loading from gsettings: easy for editing on the phone (via dconf-editor), but I doubt many will type 16 hex colors on a phone anyway