Add zsh completion support

Both bash and zsh completions could be supported using single file.

docs/shell-completion/flameshot

@@ -1,5 +1,8 @@
-# bash-completion for flameshot command
+#compdef flameshot
+
+# Shell completion for flameshot command
 # To be installed in "/usr/share/bash-completion/completions/flameshot"
+# and "/usr/share/zsh/site-functions/"
 
 _flameshot() {
 	local prev cur cmd gui_opts full_opts config_opts
@@ -64,4 +67,10 @@ _flameshot() {
 			;;
 	esac
 }
+
+if [[ -n ${ZSH_VERSION} ]]; then
+    autoload -U bashcompinit
+    bashcompinit
+fi
+
 complete -F _flameshot flameshot