Add dragIndex to drag and drop event

[mkelly95]

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[ x ] feature request
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Current behavior
Currently the drop event emits a dropIndex but a dragIndex is not emitted in either the drop of drag events

Ideal behavior
Both the drag and drop events have a dragIndex to identify the index of the dragged item in the array.

What is the motivation / use case for changing the behavior?
Enables sorting other arrays reacting to the change in the model array.

Please tell us about your environment:

• ngx-dnd version: x.x.x
  5.0.6

• Angular version: 2.x.x
  5.2.10

• Browser: [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]

• Language: [all | TypeScript X.X | ES6/7 | ES5]