[WIP] Configuration changes

TODO list:

- [x] #770: Colorscheme hierarhy.
- [x] #770: Group aliasing.
- [x] #770: Port colorschemes.
- [x] #770: Test new functionality.
- [x] #770: Update documentation.
- [x] #770: Support powerline-lint.
- [ ] #3: Add basic groups.
- [ ] #418: Configuration merging.
- [ ] #418: Test new functionality.
- [ ] #418: Update documentation.
- [ ] #418: Support powerline-lint.
- [ ] #783: Themes hierarchy.
- [ ] #783: Port themes.
- [ ] #783: Test new functionality.
- [ ] #783: Update documentation.
- [ ] #783: Support powerline-lint.
- [ ] Fix existing tests.