New features:

  * command name completion (show,insert,generate,etc)

  * `pass init <tab>` will list email addresses from gpg --list-keys

  * for 'show' command, if a folder contains a single entry, it will be
    auto-completed (recursively!)
    The other commands don't do this since you could be adding a new
    entry into an existing folder.

  * option completion (e.g., --clip)

Note: I turned off "-o filenames" because it was incompatible with the
auto-expansion. So, I instead quote using `printf "%q"` to handle files
with spaces and other odd characters.