thread-pool v1.8

• v1.8 (2021-07-28)
  • The version history has become too long to be included in README.md, so I moved it to a separate file, CHANGELOG.md.
  • A button to open this repository directly in Visual Studio Code has been added to the badges in README.md.
  • An internal variable named promise has been renamed to task_promise to avoid any potential errors in case the user invokes using namespace std.
  • submit() now catches exceptions thrown by the submitted task and forwards them to the future. See this issue.
  • Eliminated compiler warnings that appeared when using the -Weffc++ flag in GCC. See this pull request.