Fix VimPowerline.add_local_theme

After #783 it started to fail as it made Theme require defining dividers in 
theme and add_local_theme did not merge in other themes (__main__ and top).
Test that VimPowerline.add_local_theme works
Merge pull request #979 from ZyX-I/fix-add_local_theme

Fix VimPowerline.add_local_theme