Added in command checking to the dot-zshrc file to now check if things
are installed before attempting to set aliases or run programs. This
allows easier sharing of this file for other users or even other
devices.
Reviewed-on: #46
Fixed issue where when opening a second terminal it'd open in the same
tmux session making it so that it's just a mirror of the first terminal.
Closing that terminal would close all terminals. Also added an alias to
vim which calls nvim
Reviewed-on: #36
wesley