Add model tag to actor

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/154)) by Peter Mitrano (Bitbucket: [peter_mitrano](https://bitbucket.org/%7Bf79f793e-4f31-4c61-9df2-190b239e0bf2%7D/)).**

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I'd like to suggest adding a `` element within the actor element, so that it is easier to take a model you already have and script it. Currently, I copy and paste the link, joint, and plugin tags and it works as I expect, so I think sdf should be able to do that for me, similar to nested models.