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
docs: Fix building the docs without RTD theme.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Merge pull request #982 from gladiac/docs

docs: Fix building the docs without RTD theme.