Add *.opal to default inclusions

[lloeki]

.opal is an alternative extension for Opal. While it is possible to call opal files with .rb, it has side effects in Rails, so .opal is preferred in that situation.

As a workaround it is necessary to add this to .rubocop.yml:

AllCops:
    Include:
        - '**/*.opal'

Adding *.opal to default inclusions would make it work out of the box.