Add visualization plugin support

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-rendering/issues/17)) by Ian Chen (Bitbucket: [Ian Chen](https://bitbucket.org/%7Beaa6fca5-6deb-43f6-907f-971c144735dd%7D/), GitHub: [iche033](https://github.com/iche033)).**

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First step is to support a generic Scene plugin that lets user modify the existing scene.

Need to look into ign-common's [plugin framework](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-common/pull-requests/59/expanding-the-functionality-of-the-plugin/diff)