* Add a dedicated version option to CLI11 to facilitate use of version flags, similar to help flags
* add some test for the version flag
* update errors and formatting
* clear up gcc 4.8 warnings
* add a few more tests
* fix compiler error
* fix a few comments, and change default version flag to only use "--version"
* remove `version` calls and tests
* formatting and add `std::string version()` back in.