Separate emscripten build directory

This avoids the need to 'make clean' before you do a simulator build
(or vice-versa).

README.md

@@ -69,7 +69,7 @@ You may want to test out changes in the emulator first. To do this, you'll need
 ```
 cd movement/make
 emmake make
-python3 -m http.server 8000 -d build
+python3 -m http.server -d build-sim
 ```
 
 Finally, visit [watch.html](http://localhost:8000/watch.html) to see your work.