Added setExportDirectory() to MeshExporter

Currently, the MeshExporter class invariably exports to Environment.getExternalStorageDirectory() which can be undesirable in some cases.

These quick edits allow the user to set a custom directory (where I would personally use Context.getCacheDir() so I can better manage the data) to the MeshExporter object. 

See the related issue https://github.com/MasDennis/Rajawali/issues/299 for more information.