also:
- fixed parser not correctly round-tripping int format in some cases
- added `TOML_ENABLE_FORMATTERS` option
- added `operator->` to `toml::value`
- refactoring
also:
- added `formatter_flags::quote_infinities_and_nans`
- made constructors for dates/times accept any integral types
- lots of internal work on formatter base (laying groundwork for more customization)