provider: create helpers module

[flavioribeiro]

I could find some similar code on encodingcom and zencoder integration. I bet we can find more. We should create a helpers module to avoid duplicated code:

Examples:
https://github.com/NYTimes/video-transcoding-api/blob/master/provider/encodingcom/encodingcom.go#L130-L138
and
https://github.com/NYTimes/video-transcoding-api/blob/master/provider/zencoder/zencoder.go#L189-L200

https://github.com/NYTimes/video-transcoding-api/blob/master/provider/encodingcom/encodingcom.go#L324-L330
and
https://github.com/NYTimes/video-transcoding-api/blob/master/provider/zencoder/zencoder.go#L312-L316

[flavioribeiro]

another example:
https://github.com/NYTimes/video-transcoding-api/blob/master/provider/encodingcom/encodingcom.go#L340-L351
and
https://github.com/NYTimes/video-transcoding-api/blob/master/provider/zencoder/zencoder.go#L299-L312