Initial support for #770

What was done:
- Implemented loading using configuration hierarhy as proposed in the issue
- Implemented group aliasing

What was not:
- Some tests (config_reload) are failing
- Other (test_configuration) are spamming console with unexpected messages
- No support for powerline-lint
- No tests for new functionality
- Specifically I have not checked whether group aliasing actually works
- Colorschemes were not ported

Some other things: I have named this branch `config-ng` because I have other
ideas about configuration and it would be good to include them making only one
possibly backwards-incompatible merge commit instead of many. Specifically I am
going to rebase `merge-config` branch here.
Move some color defintions to colorscheme/*.json
Update documentation
Fix some problems in configuration
Make lint checker work
Make "attr" group key required, make "mode_translations" colorscheme key optional
Check for group existence for group aliases
Fix powerline-lint
Refactor some vim colorschemes to use group aliases
Fix tests