fix slow terminal init

Moved all completion.sh and .zsh files into the zsh directory, so I
don't end up scanning all of my dotfiles plus nvm each time a new
terminal is initialized.

zsh/tmux.zsh

@@ -0,0 +1,4 @@
+alias ta='tmux attach'
+alias tls='tmux ls'
+alias tat='tmux attach -t'
+alias tns='tmux new-session -s'