Add snap support

[konrad11901]

I've put together this pull request to add a snap package. I've tested it on Ubuntu 16.04, but it should work just as well on Ubuntu 14.04, Linux Mint, Manjaro, Debian, OpenSUSE, Solus, etc.

If you merge and npm run release on an Ubuntu 16.04 VM with an appropriate version of node installed it will create dist/graphiql-app_VERSION_amd64.snap.

Copy this to a Linux system, enable snap support, then run:
sudo snap install --dangerous graphiql-app_VERSION_amd64.snap. Execute it with graphiql-app from the terminal or find it in the desktop launcher.

If you create a developer account and push this to the Snap Store, it can be discovered and installed through GNOME Software and https://snapcraft.io/discover. To create the developer account, sign up here, then register the "graphiql-app" name.

You'll need the snapcraft command to push the snap file to the store.

• If you're on a Mac, you can brew install snapcraft
• If you're on Linux, it's sudo snap install --classic snapcraft

Then you can push it to the store out with:
snapcraft push graphiql-app_VERSION_amd64.snap --release stable