Proposal for adding WidgetSpan in text

[apalala-dev]

This is my first PR, I might have missed something sorry !

Some notes about the proposal :

• There are two examples folders, with one that seems outdated, I only updated one
• I added a widgets attributes which is a Map of Widget builders.
  • Someone might use this attribute and not the styles one. I don't know if styles should be required or not. At the moment, it can be filled with a blank Map.
  • Also, if someone uses the styles attribute and the widgets attribute and the same attribute is referenced in both Maps, the widgets one will win.
• I didn't do a lot of tests, this is just to give an idea on how this could be done so I didn't update the README file

[andyduke]

Already implemented in version 3.0