**Summary**
Add the actual behavior to populate `.metadata.orig_elements` during
chunking, when so instructed by the `include_orig_elements` option.
**Additional Context**
The underlying structures to support this, namely the
`.metadata.orig_elements` field and the `include_orig_elements` chunking
option, were added in closely prior PRs. This PR adds the behavior to
actually populate that metadata field during chunking when the option is
set.
**Summary**
Add `include_orig_elements: bool = True` as a new chunking option. This
PR does not implement _adding_ original elements to chunks, only
accepting this parameter as a chunking option and assigning `True` to it
as a default value when it is omitted as a keyword argument.
Note this will need to be added in other repositories as well in order
to fully support this new option by all access methods. In particular it
will need to be added in `unstructured-api` in order to become available
via the SDKs.
**Summary**
A pluggable chunking strategy needs its own local set of chunking
options that subclasses a base-class in `unstructured`.
Extract distinct `_ByTitleChunkingOptions` and `_BasicChunkingOptions`
for the existing two chunking strategies and move their
strategy-specific option setting and validation to the respective
subclass.
This was also a good opportunity for us to clean up a few odds and ends
we'd been meaning to.
Might be worth looking at the commits individually as they are cohesive
incremental steps toward the goal.
This PR culminates the restructuring of chunking over my prior
dozen-or-so commits by adding the new options to the API and
documentation.
Separately I'll be adding a new ingest test to defend against
regression, although the integration test included in this PR will do a
pretty good job of that too.
