DeepSpeech support

The use of remote speech-to-text services has some problems:
* privacy
* unavailability of the service
* need of Facebook or other free or paid accounts
* limitations of the number of requests
* more Internet traffic and latency

Is it conceivable to add support to [DeepSpeech](https://github.com/mozilla/deepspeech), which is free and can be locally hosted?

I don't know if it's mature enough to decode these captchas. I don't even know how to use DeepSpeech.

The issue names no files, tests, or entry points. First determine how the extension currently invokes remote speech-to-text services and review DeepSpeech's local integration requirements; done would require an agreed implementation scope and working local decoding for the supported captchas.

Written by the indexing model from the issue text.