Update and document Emscripten link-time options

[mpwarres]

Set INITIAL_HEAP=1MB and ALLOW_MEMORY_GROWTH=1 to reduce starting memory usage of plugins, but allow the heap to grow. Explicitly set STACK_SIZE (previously named TOTAL_STACK) to 1MiB. This is a middle ground between Emscripten's default value prior to 3.1.27 of 5MiB and its default value since then of 64KiB.

Also add comments documenting the purpose of each link-time option.