Ref Level/community#99. This is a separate ticket for leveldown because there's more to do here. Tasks:

• Create basic test workflow for linux, mac and windows
• Remove .travis.yml and appveyor.yml
• Update .npmignore (already contains .github and keeping .travis.yml and appveyor.yml is good for deps)
• Replace coveralls with codecov, update badges
• Install xvfb (on linux only) for Electron tests (and maybe make this a separate workflow)
  • This looks promising: https://github.com/GabrielBB/xvfb-action
• Add separate release workflow, triggered by tags.
  • I think I'll keep using prebuildify-ci for now; I like publishing to be manual.
• Do a test release