also:
- fixed extended-precision fractional times causing parse error instead of truncating per the spec (closes#127)
- fixed some non-spec vertical whitespace being accepted as line breaks (closes#128)
- added `format_flags::allow_unicode_strings`
also:
- fixed parser not correctly round-tripping int format in some cases
- added `TOML_ENABLE_FORMATTERS` option
- added `operator->` to `toml::value`
- refactoring