Allow use handler methods undo and redo.
Make jslint happy.
Move to API some properties, and remove default true of `immediate`

Note: labels measurements are shown regardless of the value of the
`immediate` property, so uses default of OL Measure control.
bypass code on `callbackUndo` if not exist `layerSegments`.
Update docs
Set the lost `callbackCancel`
Overridden functions `undo` & `redo` must return the result.
Adapt usage examples to focus on improvements.
Update README.md to explain the operation with undo, redo and cancel.
Update docs