CLI: Support piping and raw JSON

[mohsen1]

Piping Example:

curl petstore.swagger.io/v2/swagger.json|swagger-tools validate

Raw JSON Example:

swagger-tools validate {my-json.....}

[mohsen1]

/cc @earth2marsh

[whitlockjc]

I'll make it happen.

[whitlockjc]

I'm not sure how I'll do it just yet but here is a link that could be useful when I come back to it: tj/commander.js#137

[theganyo]

FYI. You can already do this in the swagger CLI. Here's what I did: https://github.com/theganyo/swagger-node/blob/master/bin/swagger.js#L66, although I'm not sure you need to update it here as well...

[whitlockjc]

I really just posted that link for posterity. I had it open in my browser for a week. ;)