Add support for building snaps via snapcraft

[popey]

This PR adds support for building a snap of hey using snapcraft. You can find out more about snapcraft at https://snapcraft.io/

This consists of a yaml which describes the hey snap, and a plugin for snapcraft which builds go 1.7 that is required by hey.

To build on an Ubuntu 16.04 system:-

sudo apt install snapcraft
git clone http://github.com/popey/hey
cd hey
git checkout add-snapcraft
snapcraft

This will build hey. You can install and test thus:-

snap install *.snap --dangerous

The "--dangerous" option is required when installing untrusted packages. Users installing signed packages from the store don't need that.

Once built the snap can be uploaded to the snap store as per the documentation:-

https://snapcraft.io/docs/build-snaps/publish

It would also be great to automatically push on each commit to the 'edge' channel in the store using travis or launchpad.

https://snapcraft.io/docs/build-snaps/ci-integration

[popey]

Anyone got an opportunity to test this PR and hook the pieces up to CI so we can auto-land in the snap store?