Copier

- Expanduser on all paths (so "~/foo/bar" is expanded to
"<YOUR_HOME_FOLDER>/foo/bar").
- Improve the output when running tasks.
- Remove the destination folder if the copy process or one of the tasks fail.
- Add a `cleanup_on_error` flag to optionally disable the cleanup feature.
- Add the `skip_if_exists` option to skip files, without asking, if they already
exists in the destination folder.

- Fix MAJOR bug that was preventing the `_exclude`, `_include` and `_tasks` keys from
`copier.yml` (or alternatives) to be used at all. It also interpreted `_tasks` as a
user-provided variable.

- Empty folders are now copied. The folders are also displayed in the console output
instead of just the files.
- `prompt_bool` can now have an undefined default (and answer is mandatory in that
case).
- Reactivates the `copier.yml` and `copier.yaml` as configuration files.
- The new `extra_paths` argument specifies additional paths to find templates to
inherit from.

- Back to using a setup.py instead of a pyproject.toml.
- The recommended configuration file is now `copier.toml`.

- The `copier` command-line script now accepts "help" and "version" as commands.

- Task runner 🎉.
- Use `_exclude`, `_include`, and `_tasks` keys in `copier.yml` as the default values
for the `.copy()` arguments `exclude`, `include`, and `tasks`.