Based on new bpy_extras' node_shader_utils module.

Note that this is still quiet rough on the edges, not all features
previously supported (in 2.7x) are back yet (at least missing
displacement, which should not be too hard to add, and real alpha
handling, which is probably a bit more hairy).

Also, some things like specular or mirror colors, ambient lighting, etc.
do not exists in Principled BSDF shader, tried to work around it but
there likely some room for improvements here too.