Parser does not handle URDF material gazebo extensions

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/132)) by Parker Lusk (Bitbucket: [plusk01](https://bitbucket.org/%7B54982703-ae0b-4b7e-8699-b1d2e4ae2523%7D/)).**

The original report had attachments: [model.sdf](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/132/attachments/model.sdf), [model.urdf](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/132/attachments/model.urdf)

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Using Gazebo 7.4.0 and ROS Kinetic, my URDF files that have gazebo extensions like

```xml

Gazebo/DarkGrey

```

do not convert to SDF 1.6 correctly. This seems to be a bug in the `parser_urdf.cc`. This can be recreated by running `gz sdf -p model.urdf` and noting that the `material` tag is not present.

Perhaps related to [osrf/gazebo issue #2103](https://bitbucket.org/osrf/gazebo/issues/2103/visualplugins-not-registering-sdf-elements).