install.zsh: allow alternatives in deps

Specify a dependency as a|b to check if either a or b is available as a
command. The script itself must check at runtime, which one is actually
available and use the right one. Helper methods for this are provided in
lib/common.zsh:
  - `alternatives a b` aliases a to b, if a is not available
  - `alternatives-noparam a b` replaces a with b, if a is not available,
    and discards any parameters (e.g. for replacing a formatter with cat)

lib/common.zsh

@@ -26,6 +26,14 @@ exists() {
 	type "$1" &>/dev/null
 }
 
+alternative() {
+	if ! exists $1; then eval "$1(){ $2 \$@}"; fi
+}
+
+alternative-noparam() {
+	if ! exists $1; then eval "$1(){ $2 }"; fi
+}
+
 depend() {
 	local missing
 	local i