Makefile hack for Emscripten support: goi.js

Add goi.js target to support Emscripten. Using 'emmake make goi.js'
works for me: if it starts breaking, I'll look into writing a less ugly
hack.
Add goi.html and document it

Add a minimal HTML page to load goi.js, and update README.md with
instructions on how to make it work.