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 to the MeshExporter object, but if the user doesn't set a custom directory then it defaults to Environment.getExternalStorageDirectory().

See the related issue https://github.com/MasDennis/Rajawali/issues/299 for more information.