Allow Vega charts having chart tools #111

As Vega Lite does not come automatically with useful toolbar for the generated charts, we'll may implement our own. It should be be

• customizable: users shall decide what tools to include or none a all. This is mandatory.
• extendible: users should be able to add their own tools given an icon and description. This is optional.

Minimum available tools should be

• Zoom by rectangle
• Zoom in/out/all
• Pan
• Export

See example from Plotly:

Refer also #23, where this task has been started.