Add version flag to the executable

[jkhoogland]

Hi,
it would be great if there was a --version flag available.
Is it possible to add this?
Thanks!

[lawmurray]

Thanks for the suggestion @jkhoogland, should be straightforward enough.

If anyone (including yourself) has bandwidth to add this before I do, the ideal approach would be to get the version as defined in the CMakeLists.txt file and define a macro for it (if CMake does not automatically define such a macro, I know GNU Autotools does, never done this with CMake though). The macro could then be used for the output of the program when --version is given.

The CircleCI packaging pipeline already updates the version number in CMakeLists.txt when packaging, so it should be set there already. The release tarballs on GitHub are snapshots of a tag, however, and have the version number hardcoded to 0.0.0. So will need to do something about that too. Maybe a manual update of that when releasing will be necessary, but I prefer to automate these things if possible.

[jkhoogland]

Added a PR: #64

[jkhoogland]

Updated with minimal logic for use of system yaml if available: 2521c0c

[lawmurray]

Closing now as resolved by the merge of #64. Thanks to @jkhoogland again for the addition!