Feature/ Add xls support

[yeungadrian]

PR for Issue #137

• Adding new XLS converter using same pandas -> html -> md flow, except with xlrd engine instead of openpyxl
• New test file, which copies the existing xlsx and converts it to xls
• Explicitly use openpxyl for xlsx files

[yeungadrian]

@microsoft-github-policy-service agree

[afourney]

Looks clean and simple enough. Will run tests and merge if successful.

[afourney]

Looks like some formatting errors. Can you please run pre-commit checks. Alternatively, I can run them later tonight.

[yeungadrian]

Linting errors should be resolved now

[scruel]

Thanks for creating this PR which related to my issue #137!
I already can parse most .xls files with xlrd lib, but will lose all images and some extra info in them, and some .xls files won't be able to parse with xlrd, just got errors raised. (xlrd repo is archived and won't update anymore.)
Currently, to avoid such weakness and errors, I have to first convert .xls to .xlsx, then parse it again.

FYI, I already tried many libs, but only aspose's lib can meet my requirements -- process all .xls files that I have without losing anything and raising errors:
https://products.aspose.com/cells/python-net/conversion/xls-to-xlsx/
Though this not an open-sourced lib and we are still finding other solutions.

I just thought its a Microsoft's project, so I hope maybe developers who built it can provide something better than xldr:)