Add bullet-specific tags

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/62)) by Steve Peters (Bitbucket: [Steven Peters](https://bitbucket.org/%7B2ccfed09-18b8-4921-8d58-15ef01092802%7D/), GitHub: [scpeters](https://github.com/scpeters)).**

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Bullet's featherstone implementation uses a different implementation for rigid bodies using maximal coordinates (btRigidBody) and internal coordinates (btMultiBody). We should add a tag that will hint for bullet link/joint to prefer multi body vs. rigid body implementation.

We should also add a tag for specifying the breaking force in a bullet joint.

@erwincoumans