Set up CI to handle releases and code-signing

[joshwcomeau]

Once #236/#267 is completed, we'll need to start doing this stuff on CI.

Ideally, we could just push a git tag and the CI would create signed builds.

I've heard that AppVeyor is good for Windows, and we should be able to continue using CircleCI for MacOS and Linux

Todos:

• Setup AppVeyor account for Guppy @joshwcomeau
• Create & test CircleCI publishing (MacOs & Linux)
• Create & test AppVeyor publishing (Windows)
• Setup Codesigning
  • Where to store the certificate for signing? We can store the certificate encrypted on Github as mentioned in AppVeyor how-to add secure-files.
  • Buying code signing certificate. We could get an open-source certificate at certum - or is a certificate available? @joshwcomeau

Additional context:

• Blog post about TravisCI & AppVeyor setup
• Codesigning on MacOS with-out fastlane