New
* Serve asset files (eg CSS, images, etc) from the source directory
during live preview.
* Raising `self.DoNotGenerate` in a generator will stop the current
page from being generated, without causing any errors.
Other changes
* Allow overriding an Atom feed's author and title values.
Bug fixes
* Fix wildcard generation of URLs. Some patterns were not matching, for
example `flourish generate /2021/?` was generating `/2021/index.html` but
not `/2021/04/21/index.html`, or any source with a slug starting
`/2021/...`.
* Stop live preview caching sectile-generated templates, which invalidated
the point of being able to live preview changes as they are made.
* An Atom feed without entries will still generate now.