using same color for horizontal and vertical grid
Merge branch 'v1.2' of github.com:datawrapper/datawrapper into v1.2
line-chart: disabled setting "show-bars" for all themes but NZZ, bugfixes

introduced new "option-filter" which allows to explicitly enable
visualization options that would be hidden otherwise. This way,
specific themes (such as NZZ) can introduce new options without the
other themes being affected.

I also fixed y-label placement in line charts and darkened the NZZ grid
lines a little bit..