Update post-processor.go

[ltzany]

adding --allowExtraConfig

need for certain vmx capabilities.

[ltzany]

after reviewing some of the docs.

I think i have a better approach. --allowAllExtraConfig is a better option for me but think this should be flagged (haha) if the user is building on esx or vsphere. so i will re-work this to add the flag as long as the builder is building on fusion or workstation.

i will take a deeper look at the go libraries for vmware.