* improve js_callback example
it now loads two modules:
1. imp.js
2. unimp.js
imp.js is loaded at page load whereas unimp.js is loaded when requested. A fallback is displayed during loading using Suspense
* Hash is not hardcoded anymore
* pre_build hook to build the post build hook binary
* fmt
* add .gitignore for trunk_post_build executable
* move js imports to bindings module, upadte README
