Add 'nolog' subcommand

The nolog subcommand instructs the user on how to disable logging
for the current shell. This is not (easily) doable automatically
since we can't affect a parent's environment without insane hacks.

Fixes #4

main.go

@@ -234,7 +234,7 @@ func exists(path string) (bool, error) {
 }
 
 func printUsage() {
-	fmt.Fprintf(os.Stderr, "Usage:   ./hs9001 <add/search/import>\n")
+	fmt.Fprintf(os.Stderr, "Usage:   ./hs9001 <add/search/import/nolog>\n")
 }
 
 func main() {
@@ -266,6 +266,8 @@ func main() {
 	migrateDatabase(conn, fetchDBVersion(conn))
 
 	switch cmd {
+	case "nolog":
+		fmt.Printf("Run this command to disable logging for the current shell:\n\tunset PROMPT_COMMAND\n")
 	case "add":
 		var ret int
 		addCmd.IntVar(&ret, "ret", 0, "Return value of the command to add")