Allow Html Reader to write into existing spreadsheet

[gnat42]

Sometimes you may want to read html into multiple worksheets within one
spreadsheet. Allowing the passing of a spreadsheet in makes this possible.

This is:

- [ ] a bugfix
- [x] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Its cumbersome to use the reader to create multiple worksheets within one spreadsheet from html content. This allows us to pass in a pre-existing spreadsheet object.