Add support for groups

[lukevella]

Closes #307

Example Usage

const group = slide.addGroup()
group.addText("Hello", { x: 1, y: 1, w: 3, h: 1 })
group.addText("World", { x: 2, y: 2, w:3, h: 1 })

Todo:

• Support nested groups
• Update docs

[pradeepyadav15may]

Any idea when this (grouping of widgets) will be available?

@gitbrent, @lukevella

[lukevella]

I'm going to go ahead and close this PR since it's quite stale at this point. It appears there's plans to support grouping in 4.0.0.