Theme support

[ceuk]

Porting the theme stuff over from #26 as it makes sense to split up into its own PR

(not had time to go through this yet but I will do on Sunday

• Automatically upgrades old config
• Preserves previous config
• Includes a new theme system (fixes #22)
• Adds transparency option: background passthrough if the term supports it
• Adds support for x256 colors
• normalized selected cursor length highlight in playlist screen