Copier

[All changes here](https://github.com/copier-org/copier/milestone/2?closed=1). Summary:

- Add `--prerelease` flag, which will be `False` by default. This is a behavioral
change and that's basically why I'm doing a new major release. All other changes are
minor here.
- Better docs.

[All changes here](https://github.com/copier-org/copier/milestone/12?closed=1). Summary:

- Make Copier work fine with Git 2.28.
- We have [docs](https://copier.readthedocs.io/)!
- Polish docs a little bit.
- We now run tests on macOS and Windows!

[All changes here](https://github.com/copier-org/copier/milestone/11?closed=1). Summary:

- Fix wrong templated default answers classification, which produced some questions
being ignored.

[All changes here](https://github.com/copier-org/copier/milestone/10?closed=1). Summary:

- Fix wrong prompt regression when updating.
- Remove redundant `dst` fixture in tests.

[All changes here](https://github.com/copier-org/copier/milestone/9?closed=1). Summary:

- Remove semver to avoid having 2 different versioning systems. We stick to PEP 440
now.
- Remember where an answer comes from.
- Do not re-ask to the user if already answer via `--data`.
- Support pre-migration scripts that modify the answers file.

[All changes here](https://github.com/copier-org/copier/milestone/8?closed=1). Summary:

- Templates can now use a subdirectory instead of always the template root.