Implements rowRenderer prop

[KarlaDampilag]

Issue Number

Fixes #849 (Styling specific cells based on row and column context)

Overview of PR

Implements a rowRenderer prop.

rowRenderer is a function prop that provides the following params:

• group
• timelineContext
• getLeftOffsetFromDate

These params can be used to render each row. In the demo (customRows), params are used to render "real" cells to the DOM, and style them according to group data (e.g. name), and column context (e.g. start and end times).

[KarlaDampilag]

@Ilaiwi