Add Linux snap support

[popey]

Hi! This builds upon PR #97 and closes #31

I put together this PR to add a snap package. I've tested it on Ubuntu 17.10 and 18.04, but it should work just as well on Ubuntu 14.04/16.04, Linux Mint, Manjaro, Debian, OpenSUSE, Solus, etc.

If you merge and npm run dist in an Ubuntu 16.04 VM with an appropriate version of node, it will create dist/lionshare_1.1.0_amd64.snap.

Copy this to a Linux system, enable snap support, then run:
sudo snap install --dangerous lionshare_1.1.0_amd64.snap

Run with lionshare 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 "lionshare" 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 Lionshare out with:
snapcraft push lionshare_1.1.0_amd64.snap --release stable

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