Add changes for 3.2.1

Make cxxopts.hpp ready for 3.2

Update version number in header, ready for 3.2

Fix version number

Update CHANGELOG for 3.1 release

Fix short options adding into unmatched

Fixes #312.

Update changelog for 2.2.1 release

Add to changelog for #170

Version bump 2.1.2

Add a readme comment about versions

Prepare changelog for release