processing: Add ability to specify processors per mount. The user can now specify which processors to use for each mount (i.e. each assets directory). This is mostly for disabling anything but `copy` for websites using Grunt or similar asset pipelines instead of the built-in one.

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