Add mergedefaults function that does not override existing keys
Fix typo in mergedicts_copy docstring
Check for presence of powerline_requires_* attributes

In powerline.lint.inspect their value is checked, while in other place `hasattr`
is used. This commit removes this inconsistency.
Fix errors reported by syntastic in powerline.lint.inspect
Add documentation for segment attributes
Add `powerline_segment_datas` attribute

Fixes #1035
Merge branch 'top-theme-extend' into develop