feat!: add builtin templates

WIP: document, check for breaking changes, decide on version, etc.

CHANGELOG.md

@@ -12,4 +12,11 @@ Initial release.
 - Edit, list todo notes by any date format understood by `date(1)`
 - Write custom template programs to generate content for newly created notes
 
+## v0.2.0
+
+### Added
+
+- Builtin templates for the markdown and xit formats (`.md` and `.xit`
+  extensions); the templates carry over open tasks from previous agenda notes
+
 [changelog.md]: https://changelog.md/