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Add `Rails.application.config.opal`

Accepts:
  - method_missing: `<Boolean>` (default: `true`)
  - optimized_operators: `<Boolean>` (default: `true`)
  - arity_check: `<Boolean>` (default: `false`)
  - const_missing: `<Boolean>` (default: `true`)
Update RSpec
Reset default_options before each example
Remove assets cache before each run of the spec suite
Add a warning about stale cached assets after changing opal version or config