Backer avatars in readmes

[balupton]

Update github-api, fellow, render, and projectz to render avatars for backers, such as

• Andrew Nesbitt — Software engineer and researcher
• Balsa — We're Balsa, and we're building tools for builders.
• Codecov — Empower developers with tools to improve code quality and testing.
• Poonacha Medappa
• Rob Morris
• Sentry — Real-time crash reporting for your web apps, mobile apps, and games.
• Syntax — Syntax Podcast

Image height is not customisable via markdown, and s=x should be twice that of width and height for hidpi screens. As per github sponsors listing on github profiles.

[balupton]

This is actually easy to do via https://stackoverflow.com/a/36380674/130638 as it is just https://github.com/<username>.png?size=<size> in which we have the github usernames thanks to the recent pushes in everything which keeps all urls (inc. verified username urls)