Provide custom dcgp.wasm path

[mikeheddes]

Currently dcgp.js assumes that the dcgp.wasm file is in the current directory or in the root folder of the server. I would like to let the user of this library decide where the dcgp.wasm file should be stored. This could come in handy when using this library together with webpack to create a web app.

The limitation factor in this issue is the by emscripten created glue code dcgp.js. A start has been made in the index.js file, see the commented code. This however didn't work because the generated dcgp.wasm file needs some of the generated dcgp.js code.