Ability to set DeepL API URL

[raindeer]

trafficstars

DeepL offers free limited API access with an alternative API URL: https://api-free.deepl.com/v2/translate vs https://api.deepl.com/v2/translate.

It would be great if one could change the default API URL to the free one. It's great for testing during development etc.

Manually just changing the URL in machine_translators/deepl.py works, so this should be an easy win.

[zerolab]

See #505. The PR needs a couple of tweaks and some tests. If you can help with that, please do

[vladox]

@raindeer @zerolab I'll be working on it tomorrow.

[zerolab]

fixed in #604 and will be available in the next release