Call transcribe_url_callback when callback is provided

[davidvonthenen]

Proposed changes

Reported by @jjmaldonis:
If a callback is passed to prerecorded.transcribe_url, can we automatically call prerecorded.transcribe_url_callback rather than throw an error? I think we know what the caller is intending to do, so let's do it. Same for prerecorded.transcribe_file.

Context

NA

Possible Implementation

NA

Other information

NA

[davidvonthenen]

This PR should address the problem. If it doesn't, we can reopen.
#182

Cc: @jjmaldonis