illegal chars in bone name like ':', '/' could broken Skeleton node

Godot itself has a checking for bone name when importing resources, bone name with ':' and '/' could cause the skeleton node in imported scene broken. ':' and '/' are valid characters for blender bone name, so it would be better exporter filter them out during exporting and saving users effort on changing bones name.

Relevant issue in godot-blender-exporter godotengine/godot-blender-exporter#43