Handle man page, bash completion and README generation

We generate those using cobra handlers. Note that:
* we needed to fork the bash completion mechanism to include our fixes
  for hidden commands and file completion to match
  github.com/didrocks/cobra branch-for-completion branch.
  The goal is to fix the completion issues addressed in
  spf13/cobra#983 and file completion if no match
  was found.
  A detailed upstream bug is available at
  spf13/cobra#981.
* we post-process the markdown generation to skip SEE Also and other
  irrelevant sections and add subindentation.