Collect user_id:handle mappings from JSON

[timhutton]

Minimal first-steps functionality in this PR, since there are several of us looking at this area, including: @flauschzelle, @lenaschimmel, @press-rouch, @n1ckfg

Huge thanks for the work already being shared in the discussions.

I'll do my best to synthesise everyone's thoughts but please be tolerant where I get things wrong!

Suggested roadmap (from discussion in #6):

1. existing convert_tweet() function appends whatever id:handle connections it finds to a data structure of Users. <-- this PR
2. new functionality to parse DMs, using Users, output in some simple way
3. new functionality to parse followers/followings, using Users, output in some simple way
4. new functionality to do remote lookup on Users, to improve the output of the above

[press-rouch]

Thanks for organising this into a plan. I can contribute to step 4 on the roadmap.
Edit: proposal moved to #79 (comment)

[timhutton]

Thanks @press-rouch. I've made an issue for that task: #79 - please add proposals there.