WIP

 - Simpler Rack class, less convoluted API. We want a minimal
   implementation in order to let the abstraction develop naturally
 - Move puts_columns into a class; long-term it would be good to break
   up the big method into readable chunks

TODO

 - I removed `brew list --pinned` inadvertently during the merge, re-add
   that
 - Likewise with the man page updates
 - Re-add tests that I deleted, they need to be reworked to be more
   automated and less visual ;)
 - Try to organize the control flow in `brew list` a little better, the
   nested conditionals that test the same conditions are confusing at
   first
 - The nature of columnizing the output makes it hard to appear
   responsive, in order to output the first result we have to enumerate
   all results. Still, we should try to optimize where we can