update version and copyright dates

Version 3.2.0

Version 3.1.1

- Remove unused CMake options
- Adds a single-header generator

Version 3.1.0:

- From now on, tweeny will be using a more traditional versioning scheme
- Remove some extraneous semicolons (@Omegastick)
- Adds `easing::stepped` and `easing::def` for arithmetic-like values (@ArnCarveris)
- Fix point progress calculation in multi-duration tweens (#15)
- Fix deduction of same-type values (#14)
- Use `auto` to deduce return values of operations inside various easings

Version 3

Update CHANGELOG.md

More CSS adjustments