releasenotes: add custom admonitions support for release notes By default, the extension has default sections like fix, feature, perf etc.. This patch allow user to add support for custom admonition. In order to add a custom admonition, one needs to have a .hgreleasenotes file inside the repository. All the custom directive with name specified under the tag [sections] will be usable by the extension. One important thing to keep in mind is if there exists any custom admonitions with same key as default then they will override the default ones.