Patch release FBX2glTF v0.9.3

Pre-release patch 0.9.3 of FBX2glTF brings a bunch of fixes, as well as the first means of importing PBR materials natively.

Main Feature

• Materials that were exported via the Maya Stingray PBS ShaderFX preset will be picked up as native metallic/roughness by FBX2glTF and should, if all goes well, wiggle through the series of pipes and tubes and emerge unscathed on the glTF side of things.

Fixes and Tweaks

• Instanced meshes will now be preserved as such, and no longer erroneously duplicated in the output. Many thanks to @davilovick for the fix.
• The JSON block we generate should now be ordered more sensibly. The previous (unsolicited) alphabetisation of keys has been banished.
• Blend Shape normals and tangents are now only included on opt-in command line options. It appears that they are often computed by the SDK, from the underlying geometry -- which is likely not at all what the artist wanted. That said, normal-less morph targets can look pretty terrible in their own way. Thus, we leave the choice up to you.
• We now compute glTF opacity as the inverse of the average of the RGB channels of FBX transparency. This should yield more natural results.
• We were erroneously marking some materials as transparent, when they should've been opaque.
• We no longer warn against the (perfectly valid) situation when a material lacks diffuse texture or vertex colour. This does increase the need for more information about materials when running in verbose mode; see #43.