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

[evandandrea]

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

If you merge and npm run dist -- -l it will create dist/blankup_1.1.0_amd64.snap.

Copy this to a Linux system, enable snap support, then run: sudo snap install --dangerous blankup_1.1.0_amd64.snap

Run with blankup 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 "blankup" 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 BlankUp out with: snapcraft push blankup_1.1.0_amd64.snap --release stable

(You can also push to the Snap Store programmatically from Travis.)