bake: Changes in how assets directories are configured. Change `skip_patterns` and `force_patterns` to `ignore` and `force`. Put less responsibility on the `bake` command to specify all those settings, and more on the `Baker` and `ProcessorPipeline` themselves. Add some tests.

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