sources: Make the `SimplePageSource` more extensible, fix bugs in `prose` source. The `SimplePageSource` now calls a `_populateMetadata` function that subclasses can override to add/edit their custom metadata everwhere it would be returned to the system.

[pytest]
norecursedirs = .* _darcs CVS .svn .git .hg *.egg-info dist build node_modules venv tmp* {args}