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Use /etc/hosts in addtion to $HOME/.ssh/known_hosts as the source for
hostname completion

Turn on completion caching to speed up certain comands

When completing usernames, don't include system accounts by default

The key here is not using MENU_COMPLETE which autoselects the first
entry, but using AUTO_MENU which displays the menu but autoselects
on successive tab hit.

Also, do not use zstyle menu yes because that somehow overrides
the setopts.

Only using the contents of .ssh/known_hosts when the file exists for ssh/scp auto-completion. Closes issue #20

Moving all zsh config options into a lib/ subdirectory to make way for some upcoming changes to directory structure and configuration options