fix: use configFilePath when processing stdin

[nexdrew]

As is, doing something like cat Dockerfile | dockerfilelint doesn't work because args to the processContent() function are off by one. Essentially it's passing undefined as file content, resulting in this error:

/Users/nexdrew/git/qi/dockerfilelint/lib/index.js:15
  var lines = content.split(/\r?\n/) || [];
                     ^

TypeError: Cannot read property 'split' of undefined
    at Object.module.exports.run (/Users/nexdrew/git/qi/dockerfilelint/lib/index.js:15:22)
    at processContent (/Users/nexdrew/git/qi/dockerfilelint/bin/dockerfilelint:56:31)
    at Socket.<anonymous> (/Users/nexdrew/git/qi/dockerfilelint/bin/dockerfilelint:28:5)
    at emitNone (events.js:85:20)
    at Socket.emit (events.js:179:7)
    at endReadableNT (_stream_readable.js:913:12)
    at _combinedTickCallback (internal/process/next_tick.js:74:11)
    at process._tickCallback (internal/process/next_tick.js:98:9)

Just need to pass the configFilePath in this scenario.

P. S. We would have caught this if we had unit tests for the CLI. I will work on putting something together to address this in the near future. 👍

[coveralls]

Coverage remained the same at 98.886% when pulling 3bde4f3 on nexdrew:fix-stdin into d3c8346 on replicatedhq:master.