Publish prebuild binaries

[felixfbecker]

Compiling node-oniguruma currently takes up a major part of install & total build time in CI. It would be great if pre-built binaries for the usual targets were uploaded to GitHub with https://www.npmjs.com/package/prebuild and then defaulted to if available with https://github.com/prebuild/prebuild-install (falling back to compiling like today).