Modify the client HTML file when copying it to the build directory to
apply current CLIENTBIN and BASEGAME. It always loads engine/data from
the current directory (no need to try to locate the build directory).
This enables several things:
* Optionally load pk3 files from a web server at runtime instead of bundling them with Emscripten at build time
* Set command line arguments via URL param
* It's not ugly
