Add snacraft config

This allows the easy generation of snaps as requested in #227.

This commit does not add automatic generation and publication of snaps
to the Snap Store, but they can easily be generated for local/personal
use by

```
snapcraft
sudo snap install gutenberg_xxx.snap
```

Add directories with snapcraft artifacts to `.gitignore`.

.gitignore

@@ -13,3 +13,9 @@ huge-kb
 
 current.bench
 now.bench
+
+# snapcraft artifacts
+snap/.snapcraft
+parts
+prime
+stage

snap/snapcraft.yaml

@@ -0,0 +1,21 @@
+name: gutenberg # you probably want to 'snapcraft register <name>'
+version: '0.3.3+git' # just for humans, typically '1.2+git' or '1.3.2'
+summary: A fast static site generator in a single binary with everything built-in.
+description: |
+  A fast static site generator in a single binary with everything built-in.
+  https://www.getgutenberg.io
+
+grade: devel # must be 'stable' to release into candidate/stable channels
+confinement: strict # use 'strict' once you have the right plugs and slots
+
+apps:
+  gutenberg:
+    command: gutenberg
+    plugs:
+      - network
+      - network-bind
+
+parts:
+  gutenberg:
+    plugin: rust
+    rust-channel: stable