[usdMaya] Loading the shading mode plugins.

[sirpalee]

This PR is a general improvement for the shading mode exporter plugin interface.

• Previously there was no functionality to load the plugins. I added a function similar to the primWriter plugin loading code, that's run before any of the exporters are accessed.
• The default values caused a bit of confusion. The GPrim Colors -> displayColor remapping was missing from the export code (both the translator and the usdExport call).
• A request to an invalid export appended empty exporter to the list with the same name. This problem was fixed by accessing the exporter through an iterator rather than std::map's operator[].
• If the manual export command was used instead of the file translator, the shading mode worked incorrectly. The shading modes were accessible by their internal names, which are not documented anywhere. I added support for the custom names on the file/export interface.

[jtran56]

Filed as internal issue #147855.

[sirpalee]

Note the original commit mixed up GPrim Colors and Material Colors shading mode, and exported them incorrectly. The PR was updated.