Add Dynamic valueBinder Property to Spreadsheet and Readers

[oleibman]

Fix #1395, a 2020 issue which had been marked stale and is now re-opened. Static valueBinder property of Cell isn't ideal. It would be more flexible to make it a dynamic property of the spreadsheet. Static property will continue to be used, but dynamic property will be used first if it is set. Readers will also be changed to add a valueBinder property which they pass to the spreadsheet; however, it will make a difference only for Csv/Html/Slk, since the other readers use setValueExplicit which ignores valueBinder.

Documentation is updated in several places to note that dynamic property is preferred over static.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• 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 and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.