Feature Reporting.

This is based on the conversation in #8 and https://github.com/mintty/mintty/issues/881

And further development in these documents:

https://docs.google.com/spreadsheets/d/1H4OK8z98ptySnAPjlej56lrL6b0S1j6okwp7KLRTDBQ/edit#gid=0 https://docs.google.com/document/d/1Is0xKlJTyhuPBVypzJHgNtKsZkStoI2WTLfiVSp6RC4/edit