added source-map-loader

[Place1]

I've added source-map-loader to the webpack config for development.
This allowed me to successfully use all the VSCode debugging goodness!

Here's the VSCode launch target I made:

{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "chrome",
      "request": "launch",
      "name": "Launch Chrome",
      "url": "http://localhost:3000",
      "userDataDir": "${workspaceRoot}/.vscode/chrome",
      "linux": {
        "runtimeExecutable": "chromium-browser"
      },
      "webRoot": "${workspaceRoot}",
      "sourceMaps": true,
      "sourceMapPathOverrides": {
        "webpack:///*": "${workspaceRoot}/*"
      }
    }
  ]
}

I saw you working on a CRA 1.0 update which involves a webpack upgrade to v2. I'm fine if you want to wait on this one and I can update the patch after that PR is done. #61

[wmonk]

hey @Place1, thanks for this! Have merged in cra@1.0.4 now, so please can you rebase your PR against that!

The tests are all working fine now, so please run yarn run e2e at the root of the repo before you push.

[Place1]

hey @wmonk I think that's all done now. Tests pass on my machine.