If test in `make test-cmd` is failing, now it will show diff so that
it will be easy to debug.
for example,
```
===> Starting test <===
convert::expect_success_and_warning: Running: 'kompose -f /home/snarwade/go/src/github.com/kubernetes-incubator/kompose/script/test/fixtures/service-name-change/docker-compose.yml convert --stdout -j' expected_output: '/home/snarwade/go/src/github.com/kubernetes-incubator/kompose/script/test/fixtures/service-name-change/output-k8s.json' expected_warning: 'Unsupported root level volumes key - ignoring'
WARN Unsupported root level volumes key - ignoring
WARN Unsupported depends_on key - ignoring
103c103
< "value": "bitnami_wordpres"
---
> "value": "bitnami_wordpress"
FAIL: converted output does not match
```
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
