Allow to change parent container for the widget v6 feature

[miniben-90]

• Please check if the PR fulfills these requirements

• The PR is against the development branch
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...). If this is a fix, please tag a bug.
  This is a feature that allows you to change the target container instead of the body element for the widget generation.

• What is the current behavior? (You can also link to an open issue here)
  The component generate the widget as a append child of body but in shadow DOM it can be problematic in case if the styles not loaded in the main DOM.

• What is the new behavior (if this is a feature change)?
  Add a new parameter container to define a HTMLElement target instead of body.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  Not a breaking change. If the parameter's not defined it'll use body as target.

• Other information: