Provide tooltip support

[nesterone]

• option to show/hide tooltip
• slots to customize it's content

[nesterone]

About tooltip #36, my initial idea was to use vue slots, however, it's maybe a sugar API in future

In general vue-vega just a wrapper around Vega/Vega-Lite library, so I think it makes sense to use the same approach as vega-embedded:

``js
var handler = new vegaTooltip.Handler(options);
VueVega.use({tooltip: handler.call, mode: 'vega-lite'})

[xmnlab]

@nesterone

currently, after #44, it is possible to use tooltips using spec. I think also it is possible use it by specifying tooltip on encoding. should that be sufficient?

[chris-canipe]

I'm having trouble with the VueVega.use example above. Has anyone got tooltips to work in this manner? They work if specified in encoding, but I want them to be automatic, similar to Voyager.