cmd: add a 'version' subcommand and flag to show the version

ops/compile.sh

@@ -1,3 +1,10 @@
 #!/bin/sh
 
-go build -tags fts5 -o gas ./cmd
+# Stamp the version into the binary.  `git describe` fails when this isn't a git checkout (e.g. a
+# source export with no .git dir); in that case leave it empty, and pkg/version falls back to the
+# build info that the Go toolchain embeds.
+VERSION=$(git describe --tags --always --dirty 2>/dev/null)
+
+go build -tags fts5 \
+    -ldflags "-X git.offline-twitter.com/offline-labs/gas-stack/pkg/version.Version=$VERSION" \
+    -o gas ./cmd