Cobra, the CLI commander in use in Hugo, has some long awaited improvements in the error handling department.
This enables a more centralized error handling approach.
This commit introduces that by changing all the command funcs to `RunE`:
* The core part of the error logging, usage logging and `os.Exit(-1)` is now performed in one place and that one place only.
* The usage text is now only shown on invalid arguments etc. (user errors)
Fixes#1502
Add a new command, genautocomplete, wich generates a Bash completion script (zsh and others later).
The script is by default written to `/etc/bash_completion.d/hugo.sh`; this can be set in `--completionfile=/some/file`.
Fixes#438
