Attach the class "baseline" to gridlines that are at x=0 or y=0, so
they can have additional styles.
Close #712
[minor] Rename ".gridlines .baseline" to ".gridlines .zeroline" as it is
more clear where it is applied and disambiguates from AreaPlot/BarPlot baseline
Merge pull request #738 from palantir/baseline-gridlines

Attach the class "baseline" to gridlines that are at x=0 or y=0, so