zsh:alias:add_flags: Keep existing aliases

Instead of overwriting existing aliases, reuse them.
This makes it possible to add multiple flags in multiple calls to
`add_flags` for example when a flag should only be added on certain
systems, etc..

.config/zsh/zshrc.d/30-alias.zsh

@@ -10,7 +10,7 @@
 	add_flags() {
 		(( $# >= 2 )) || (( $+commands[$1])) || return 0
 
-		alias "$1"="$*"
+		alias "$1"="${aliases[$1]:-$1} ${*[2,-1]}"
 	}
 
 # Default flags