feat(unstructured): adding element index as metadata when partitioning by elem

[lambda-science]

Is your feature request related to a problem? Please describe.
With unstructured we can do partition: one doc per file, one doc per page or one doc per elem.
When doing one doc per elem there is no way to track orders of element coming from a same doucment.
This informations can be usefull for example for a ContextExpander component.
Let's say you retrieve an element, you can retrieve also the previous and next element to expand the current context .

Describe the solution you'd like
Just automatically add index metadata.
I will do a PR.

[anakin87]

done in #382
new release: https://pypi.org/project/unstructured-fileconverter-haystack/0.4.0/