implement storing to dir/file, add tests

When kompose convert -o artifacts is run, then if artifacts is a directory,
kompose stores all the resulting files inside it, and if it's not a directory,
it stores the artifacts in a single file.

Fixes #209

cli/command/command.go

@@ -154,7 +154,7 @@ func commonConvertFlags() []cli.Flag {
 	return []cli.Flag{
 		cli.StringFlag{
 			Name:   "out,o",
-			Usage:  "Specify file name in order to save objects into",
+			Usage:  "Specify path to a file or a directory to save generated objects into. If path is a directory, the objects are stored in that directory. If path is a file, then objects are stored in that single file. File is created if it does not exist.",
 			EnvVar: "OUTPUT_FILE",
 		},
 		cli.IntFlag{