update .npmignore to allow sourcemaps to reference the source files

[r14c]

This change was raised before, but I'm going to try again. This is still one of the better event source clients out there imo.

Closes #18

Currently we are using set ignoreWarnings option to skip the parse error. https://webpack.js.org/configuration/other-options/#ignorewarnings

module.exports = {
  // ...
  ignoreWarnings: [/Failed to parse source map/]
}