Formatter flag to force multi-line arrays

[rafal-c]

What does this change do?

This PR introduces a formatter flag force_multiline_arrays which makes all arrays printed as multi-line (one element per line), which may increase readability. In master, only large enough arrays are printed multi-line and there is no control over it.

This change is fully backwards compatible.

Is it related to an exisiting bug report or feature request?

No, as far as I know, this has not been requested yet.

Pre-merge checklist

• I've read CONTRIBUTING.md
• I've rebased my changes against the current HEAD of origin/master (if necessary)
• I've added new test cases to verify my change
• I've regenerated toml.hpp (how-to)
• I've updated any affected documentation
• I've rebuilt and run the tests with at least one of:
  • Clang 8 or higher
  • GCC 8 or higher
  • MSVC 19.20 (Visual Studio 2019) or higher
• I've added my name to the list of contributors in README.md

[marzer]

Looks good,, thanks :D