version 3.1.0

Added

• support unreleased toml feature (with TOML11_USE_UNRELEASED_TOML_FEATURES)
  • Leading zeroes in exponent parts of floats are permitted.
  • Allow raw tab characters in basic strings and multi-line basic strings.
• add utility accessor function toml::value::at
  • val.as_table().at("key") -> val.at("key")
  • val.as_array().at(0) -> val.at(0)

Fixed

• Fix example code in README.md (@jcmoyer )
• Fix conversion requirements
• Warnings about rvalue references in clang