Add update command.

lib/commands/update.sh

@@ -0,0 +1,28 @@
+update_command() {
+  local update_to_head=$1
+
+  (
+  cd $(asdf_dir)
+  pwd
+
+  if [ "$update_to_head" = "--head" ]; then
+    # Update to latest on the master branch
+    git checkout master || exit 1
+
+    # Pull down the latest changes on master
+    git pull origin master || exit 1
+    echo "Updated asdf to latest on the master branch"
+  else
+    # Update to latest release
+    git fetch --tags || exit 1
+    tags=$(git tag)
+
+    # Pick the newest tag
+    tag=$()
+
+    # Update
+    git checkout "$tag" || exit 1
+    echo "Updated asdf to release $tag"
+  fi
+  )
+}