Offer .tar.gz releases

The zip/unzip commands aren't installed by default on most Linux
distros. However, tar is almost always installed. Additionally, users
may get confused when they try to uncompress an archive with
gzip/gunzip, only for it to fail because gunzip is for .gz files, not
.zip.

This change makes it a little easier to grab a copy of a nats-server
release binary.

.goreleaser.yml

@@ -52,6 +52,13 @@ archives:
     files:
       - README.md
       - LICENSE
+  - name_template: '{{.ProjectName}}-{{.Tag}}-{{.Os}}-{{.Arch}}{{if .Arm}}{{.Arm}}{{end}}'
+    id: targz-archives
+    wrap_in_directory: true
+    format: tar.gz
+    files:
+      - README.md
+      - LICENSE
 
 checksum:
   name_template: 'SHA256SUMS'