Series of misc improvements

[eirannejad]

• marked api as public
• glTF exporter now accepts a config object instead of arguments
• exporting non-standard elements is now optional glTFExportConfigs.IncludeNonStdElements
• filename is only the main file name and not the full path.directory will contains all the files.
  • in single binary, the is one file called <filename>.bin
  • in multi file, there is one <filename>.gltf and a series of *.bin buffers