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:
- added `date_time` converting constructors from `date` and `time`
- added `is_key<>` and is_key_or_convertible<>` metafunctions
- exposed `TOML_NAMESPACE_START` and `TOML_NAMESPACE_END` macros to help with ADL specialization scenarios
- added encoder and decoder for `toml-test` (closes#125)