Store selected theme in config

[kepi]

Currently, selected theme is not stored anywhere in base16-manager which was OK so far.

Here are some use cases where it can be needed in future:

• changing directory locations like Move base16-manager config/data dir to correct (XDG) location #46
• we need to re-set theme on template update (format change etc)
• user want to see current selected theme
• we may decide in future to set themes per template Support different theme per template #49

Another approach might be not to store selected theme but implement logic in template support code to detect current theme. Upside would be awarenes of manual changes. Downside may be that user may depend on us to set theme he choose so we should be source of truth for themes :)

If #49 is against project directions, we can store selected theme in single file in config home. If #49 can happen sometime, it would be better to decide how to store default selected, overwrites etc (text file, multiple files, yaml config...)