Fix code path regression

To make sure that sub_dirs and deps are properly added to the code path
regardless of -r/--recursive, add 'preprocess' and 'postprocess' to the
list of recursive commands. Without this fix, you'd have to, for
example, run 'rebar -r ct skip_deps=true', if you wanted to run
base_dir's ct suites with deps and sub_dirs on the code path (while
skipping all non-base_dir ct suites).