Do not advertise unwanted support for filetypes
Sometimes applications do not want to support filetypes that SMTK supports (e.g., aeva does not want smtk::mesh to advertise its support for STL files, VTK data — see aeva/aeva#43 (closed) , aeva/aeva#47 (closed) ). SMTK should provide applications with a way to either remove registered reader/importer functionality by filetype or explicitly add filetypes.