resolves #5 add captioned title on charts

[ggrossetie]

Includes a major refactoring 🤯

What I Did

• Introduce a concept of registry with a list of chart providers
• Create and return a ChartBlock (which extends Asciidoctor::Block) from the extensions
• Register a convert_chart method on the converter (it supports the built-in Html5Converter, CompositeConverter and TemplateConverter)
• Each chart providers/converters declare a list of chart type they can handle via the handles? method
• Each chart providers/converters declare a unique engine name via the Asciidoctor::Chart::Registry.register method