Allow notifications

Previously any notifications add with notify would be ignored by Buildpipe, this PR allows you to send a notification at the end of a build.

Fix buildpipe not working if any attributes are used on the wait step

Support BuildKite attributes on the wait step: https://buildkite.com/docs/pipelines/wait-step, to fix this error:

422 env is not a valid property on the wait step. Please check the documentation to get a full list of supported properties.

- env: {}
  if: build.source == "schedule" && build.branch == "master"
  wait: null

Bump version to 0.10.1

The git tag for 0.10.0 is pointing at the wrong commit.
This is causing the hook to still point at the previous version.
We could fix the tag, but that doesn't necessarily propagate cleanly.
Instead, let's just use a new version number to fix this.

Manually specify build projects

1. Add support for manually specifying build projects

Fix depends_on

1. Fix space-separation in step keys.
2. Fix parsing when depends_on is a string instead of an array.

Add support for unique step keys and depends_on

1. Generate unique keys for project steps, and add support for depends_on.

Project-wide environment variables

1. Add project-wide environment variables which are applied to all steps

Capture error message on pipeline upload failure

1. Capture and log stderr on Buildkite pipeline upload failure.

Rewrite in Go

1. Rewrite plugin in Go
2. Change license to Apache 2

Update pipeline and plugin schema

Changes:

1. Projects are now defined in the dynamic_pipeline.yml file and not the initial pipeline. This allow define most configuration changes in one file.
2. The diff command has been split into two commands to give more flexibility on whether you are specifying the default branch or a non-default branch.