Breaking changes: the built-in `angular` and `keepachangelog` templates
were refactored into standalone files using Jinja macros. The templates loading
mechanisms have therefore been updated to match this change. It means
templates folders (containing a `changelog.md` file) are not supported anymore.
This change makes it possible to create and use custom templates more easily,
as we don't expect a particular file name anymore (`changelog.md`).