Reusable Commands and Executors, Xcode 10 Image, and macOS Plan Settings Updates

CircleCI 2.1 config keys are now available for reusing commands, executors, and parameters to simplify your .circleci/config.yml file. See the Reusing Config document for examples and instructions. Refer to the Configuring CircleCI reference for 2.1 syntax requirements.
We have released the Xcode 10 image on CircleCI 2.0. Add the following to your .circleci/config.yml file to select that version of Xcode in your jobs:
```
macos:
xcode: "10.0.0"
```

Note: A known issue, for which a fix is in-progress, prevents shipping iOS apps built on the Xcode 10 image to the App Store.

For Cloud Container macOS plans, there is an improved flow for adding containers on the Plan Overview page. This can be found under Settings > Plan Overview.

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