zsh: switch back to emacs keybindings

Vim keybindings in the shell work okay, but the lack of a status line
slowed me down quite a bit. And the lack of ctrl-r for history searching
wasn't great. Maybe I'll go back to Vim keybindings later once I figure
out a good way to solve these issues, but for now it's back to the
default emacs keybindings.
tmux: enable ctrl-a emacs keybinding

``ctrl-a`` in emacs keybindings for shell move to the start of the line. But
since I switched to using ``ctrl-a`` for the tmux prefix, ctrl-a doesn't
work. So instead I can now use ``ctrl-a a`` inside tmux