Introduce 'post' and 'pre' commands

Both post and pre are called after all 'cmd' are already called, but
output from 'pre' is put on top of configuration output and output from
'post' is put on bottom of confguration output.

Rationale is dynamic configuration, where you'll only know after running
some commands what commands you want to run.

Both except blocks, such as

pre do
  # commands to execute
end

Both can be called multiple times

0 files changed, 0 insertions, 0 deletions