zsh: Add flags for du

This changes the output to be human-readable.

Additionally du will now only check 2 levels deep (can be overridden by
explicitly setting `-d <depth>`).

.config/oh-my-zsh/aliases.zsh

@@ -32,6 +32,7 @@ alias rm='rm -i'
 ## adding flags
 alias df='df -h'
 alias free='free -m'
+alias du='du -h -d 2'
 
 ## color and condense ip output
 alias ipa='ip -br -c addr show'