Add custom tag callback

[Sub6Resources]

Some use cases may require custom tags to be rendered. Add a callback to the main Html widget that is called every time an unknown element is encountered. The programmer can then return their own widget for their custom elements.

[Sub6Resources]

Here is an example of what the callback might look like:

Html(
  data: myHtmlWithCustomTags,
  customTagCallback: ((element) {
    switch(element.localName) {
      case "my-custom-tag":
        return MyWidget(
          text: "Lorem ipsum dolor sit amet",
          onPressed: (() {
            //...
          }),
        );
    }
    return Container();
  }),
)

[Sub6Resources]

Also, the callback would be able to override a tag and add its own implementation (as in #21 ):

Html(
  data: myHtml,
  customTagCallback: ((element) {
    switch(element.localName) {
      case "code":
        return MyCustomCodeWidget(
          code: element.innerHTML,
        );
    }
    return Container();
  }),
)

[Sub6Resources]

This is currently being added in #23 and will likely be added as version 0.7.2

[Sub6Resources]

Added in version 0.8.0