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.