homebrew: Add 'brew upgrade' alias for cask

This adds alias for 'brew upgrade --cask' specifically for casks
via alias 'casku' (analogous to 'brewu').

modules/homebrew/init.zsh

@@ -43,4 +43,5 @@ alias caski='brew install --cask'
 alias caskl='brew list --cask'
 alias casko='brew outdated --cask'
 alias casks='brew search --cask'
+alias casku='brew upgrade --cask'
 alias caskx='brew uninstall --cask'