Extended types of dependencies which posts objects can return.
Post.extra_deps cannot be called from outside anymore.
Next step is to rewrite the functions in Post to allow the page compiler to send extra dependencies (instead of reading them from the Post object) and to add functions which allow to add additional dependencies (via callbacks or directly).
Forgot to add lang parameter.
Moved extra_deps() logic from post.Post to plugin_categories.PageCompiler.
Renamed rest_deps function, and added language parameter.
Made extra dependency system compatible to old extra_deps system.
Added facility to Post object to inject dependencies.
Changed PageCompiler's register_extra_dependencies to get_extra_dependencies, which uses the new utility functions.
Refactored PageCompiler.register_extra_dependencies: split into two functions.
PageCompiler.register_extra_dependencies now emits a warning.
All page compilers in core overwrite this function (so no warning will be displayed).
Moved .dep logic completely into ReST compiler.
Removed default warning for PageCompiler objects who don't override register_extra_dependencies.
Consequently removed overrides of register_extra_dependencies in core compiler plugins which don't do anything.
Fixed 'fake compiler' for RSS tests.
Fixed a problem in the interplay of doit and config_changed when a config_changed object appears in more than one uptodate list of a task. (See discussion in #1529.)
Fixed a typo.
Merge pull request #1529 from felixfontein/post_dependency_extensions

Refactoring post dependency handling to allow easier injection of dependencies.
Adding support to write other metadata (for example 'category', or some information coming from the original format which has no correspondence in Nikola but is worthwhile preserving).
Using **kwargs instead of other={} to specify other metadatas.
Making it possible to avoid rewriting HTML in ImportMixin.write_content.