Set up CI builds

[huntie]

Configure automated test builds on a CI provider, ideally for each supported platform.

Locally, I have written unit tests designed to run with Sublime's UnitTesting package, for which this reference configuration exists to configure Linux and macOS builds with CircleCI.

In addition to UnitTesting, the tests depend on the snapshottest library. On my local Ubuntu machine it has been sufficient to install this globally using pip and everything has run correctly when starting tests from within Sublime. However, I have had no luck as yet with the Linux build in a remote CircleCI container, as tests are run in a context where this dependency cannot be resolved.