Add support for display option

Ref #418
Add MarkedDict and MarkedList types
Improve handling of marked types in powerline.lib.mergedicts
Implement configuration merging

Fixes #418

TODO:
- Specific tests.
- Better handling from powerline-lint side (it should no longer check separate
  files for themes and colorschemes).
- Better handling for marked values in powerline-lint when using merging.
Some indent/punct position changes