Allow inclusion of links, collisions, and visuals

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/75)) by Nate Koenig (Bitbucket: [Nathan Koenig](https://bitbucket.org/%7Bc862cdd9-fcc8-4419-9fc7-e20db14b8fcb%7D/)).**

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Right now `model://...` only facilitates inclusion of a whole model.

We should add in the ability to include specific links, collisions, or visuals from a model.

For example, to include a specific collision:

```
#!c++

model://MODEL_NAME/LINK_NAME/COLLISION_NAME

```

Start by tracing the existing handling for model:// URIs and how links, collisions, and visuals are represented in the parser. Use the example URI as the target behavior, then determine how including each specific element should be represented and validated. Done means specific links, collisions, and visuals can be included without breaking whole-model inclusion.

Written by the indexing model from the issue text.