Prevent binding from panicking when not in wasm

[zyllian]

This fixes an issue I ran across while using the same template code to do SSR and DOM rendering where using value:bind would panic during SSR because it tries to call an unimplemented function from wasm_bindgen.

This isn't a particularly elegant solution to the problem, so I'm sure it can be improved.

[lukechu10]

LGTM. Thanks for the contribution!

[arctic-hen7]

Indeed, this will make using data binding possible in Perseus!

[lukechu10]

Ah, the tests are failing because the #[cfg]s are in the wrong place. These are evaluated during compile time so the target is the target on which the code is being built. The #[cfg]s should belong inside the quote! macro instead.

[zyllian]

Okay, that should fix it and also make the cfg attribute less restrictive.