Feature request: Add new EM_LOG_FILE environment variable

When working on diagnosing Emscripten behavior when integrated to a complex build system, I find myself hacking emcc.py often to change the Python logger to log all output to a file.

I wonder if it would make sense to make this a feature? E.g.

```
set EM_LOG_FILE=C:\code\emcc.log
set EMCC_DEBUG=2
run_complex_build_system.exe
```
would help dump all verbose Emscripten log to file easily. In our case, `run_complex_build_system.exe` spawns emcc multiple times, but does not show the EMCC_DEBUG contents on stdout.

I can do the impl. if the idea gets green light.