Add a snap package (https://snapcraft.io)

[popey]

Hi! I put together this PR to add a snap package. I've tested it on Ubuntu 17.10 and 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 it will create dist/ling_0.2.0_amd64.snap.

Copy this to a Linux system, enable snap support, then run: sudo snap install --dangerous ling_0.2.0_amd64.snap

Run with ling or find it in the 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 "ling" 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 Ling out with: snapcraft push ling_0.2.0_amd64.snap --release stable