Use `try/except KeyError` in place of `if in dict/else`
Pop path from self.loaded on exception
Add support for display option

Ref #418
Add mergedictcopies function
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.

Note: to be split into separate branch.
Make vim and ipython ext configurations optional
Fix typo regarding priority handling
Special-case .copy() method

Still more fixes to add for powerline-lint