Add a child object widget for displaying text or other info in center of gauge.

[TommyLCox]

Related to #1 but not in labeling actual segments or ticks, more so just to be able to add other info to a gauge by giving it the ability to have a child widget.

A first pass at this is on this fork, https://github.com/AdvancedSystemsUnlimited/flutter_gauges with docs at the end of that Read.me and example usage in the example.

[Munsif-Ali]

@JulianAssmann any progress?