Add .npmignore

You don't have a [.npmignore](https://www.npmjs.org/doc/misc/npm-developers.html#keeping-files-out-of-your-package) file so npm is using the `.gitignore`. The compiled output from the pre-publish task is getting axed because of this which makes it difficult to use esri-leaflet with dependency managers such as browserify which expect the `main` file specified in `package.json` to exist.

This pull-request contains a `.npmignore` file which is a duplicate of the current `.gitignore` without the `dist/**/*.js` exclusion.
You will need to re-publish to npm after merging this.