unknown variables in argument array not an error if not used

[nicklan]

This is more a divergence from go-jsonnet, as one could argue that the lazy nature of jsonnet should mean that this works.

test:

std.format("%s", ["a", b])

go-jsonnet:

$ jsonnet unk.jsonnet
unk.jsonnet:1:24-25 Unknown variable: b
std.format("%s", ["a", b])

sjsonnet:

$ sjsonnet unk.jsonnet
"a"