v1.0-beta.17

We skipped v1.0-beta.16, sorry about that 😎

• BUIDLKITE_S3_ACCESS_KEY and BUIDLKITE_S3_SECRET_KEY are now supported when using a custom S3 bucket as an artifact upload destination
• Added a new pre-artifact hook
• SHA-1 checksums are uploaded to Buildkite for each artifact
• Added a new subcommand buildkite-agent artifact shasum that will output to STDOUT the SHA-1 checksum of an artifact
• Don't output terminal colors on Windows or if the output isn't a TTY
• All logging now goes to STDERR by default
• Ignore ErrTooLong errors when scanning lines
• Reset bash shell options after running a hook or a build script
• Allow agents to be remotely shut down