Integrate emrun with noInitialRun / INVOKE_RUN=0

Some "powerful" Web APIs are only allowed in response to user interaction (e.g. click on a button) somewhere on the call stack.

In these cases, it's useful to avoid running Emscripten automatically, and instead invoke it manually in response to such allowed user interaction.

`noInitialRun` / `INVOKE_RUN=0` allow to build code like this, but they're not very convenient to use with the autogenerated HTML and `emrun` helper during testing and development, because it will simply open HTML and the only way to run the code is to open DevTools and call `callMain` manually.

I propose that that, when code is compiled in such mode, HTML would be generated with a simple "Run" button which will invoke `callMain` on click with command-line arguments from GET params just like in a regular build with auto-run.