`stdout` will be printed if no option for output is specified

[surajnarwade]

Resolves #80

[surajnarwade]

@kadel,

$ opencompose convert -f hello-nginx.yml -o .
created file "helloworld-service.yaml"
created file "helloworld-deployment.yaml"

it's working :)

[surajssd]

@surajnarwade please update the commit message and title of the PR as well, it is very confusing what change this PR adds.

These might help you:

• https://chris.beams.io/posts/git-commit/
• http://conventionalcommits.org/

[surajssd]

I always refer those before I send a PR! :-)

[surajnarwade]

@kadel @surajssd @containscafeine needs review for this