Don't require installing wasm-bindgen-cli globally

[dsherret]

Global commands are a huge headache because version differences cause build failures (wasm-bindgen-cli gives a very unhelpful message when this occurs). It would be nice if this didn't require having to install wasm-bindgen-cli globally.

[dsherret]

This would supersede #4 (Edit: I think... or maybe that would still be necessary for ensuring the wasm-bindgen specified in the cargo.toml is the same as what's in wasmbuild)