Change Webpack configuration for external Flickity dependency to fix require issues on Linux

[LuudJanssen]

Hey! This fixes #87. I used my local Git username instead of my Github username by accident, but it's me :)

I also added the use of the cross-env dependency, since the environment variables weren't being set correctly in the build script on my Windows machine.

I also added the Node *.gitignore template to not include the "coverage" folder and also excluded the ".idea" for Intellij users.