This PR surfaces the configuration options available to updates, previews, and destroys to the Pulumi Service. As part of this I refactored the options to unify them into a single `engine.UpdateOptions`, since they were all overlapping to various degrees.
With this PR we are adding several new flags to commands, e.g. `--summary` was not available on `pulumi destroy`.
There are also a few minor breaking changes.
- `pulumi destroy --preview` is now `pulumi destroy --dry-run` (to match the actual name of the field).
- The default behavior for "--color" is now `Always`. Previously it was `Always` or `Never` based on the value of a `--debug` flag. (You can specify `--color always` or `--color never` to get the exact behavior.)
Fixes#515, and cleans up the code making some other features slightly easier to add.
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