Improve visual_range segment
Do not use “rows” and “cols” in text

Reason: string “1 rows” is incorrect and I do not want to implement proper 
number handling (*proper* handling is *not* limited to plural/singular form like
in English).
Add tests for visual_range
Add fix for old Vims where getpos does not return list of integers
Merge pull request #957 from ZyX-I/better-visual-range

Improve visual_range segment
Only check that all colorschemes have theme, but not the opposite
Implement theme hierarchy

Fixes #783
Move all unicode characters from themes to powerline.json
Make now_playing segment accept state symbols from arguments
Implement single-level merging of “args” dictionaries
Copy a number of unicode values to powerline.json
Allow checking NowPlayingSegment