add exm specs, update iqm spec with clarifications

[shakesoda]

iqm's spec leaves some ambiguity which needs to be cleared up, especially by leaving the coordinate system unspecified (i.e. "use blender's").

exm should actually specify that it uses blender's z-up. iqm spec text should be clarified for what it actually does right now.

will need to begin to formalize the metadata block

[shakesoda]

added the iqm specs with a couple clarifications, i'm sure more would be useful.

exm json is still unspecified, don't rely on it being anything specific (it's currently disabled, regardless)