Need support for callbackUrl parameter when uploading (pushing) files to Smartling

[enegron-sl]

Smartling file upload feature / API supports a callbackUrl parameter. This is not a "directive" and so needs to explicit support as a parameter to the upload / push feature.

smartling-cli files push file.txt --callback "https://smartling-api.site.com/"

where the value of callback is any URL the user wants to specify to receive callback on file translation completion.

DoD:

• Add new optional --callback for files push command
• Add new parameter in config (smartling.yml) for push section