Add support for version specifications

This commit adds a `#[wasm_bindgen(version = "...")]` attribute support. This
information is eventually written into a `__wasm_pack_unstable` section.
Currently this is a strawman for the proposal in ashleygwilliams/wasm-pack#101

crates/shared/src/lib.rs

@@ -22,6 +22,7 @@ pub struct Program {
 #[derive(Deserialize, Serialize)]
 pub struct Import {
     pub module: Option<String>,
+    pub version: Option<String>,
     pub js_namespace: Option<String>,
     pub kind: ImportKind,
 }