feat: implement version flags

This moves the version command to the dokku binary, and also implements both -v and --version flags on the binary.

tests/unit/10_version.bats

@@ -0,0 +1,28 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+setup() {
+  global_setup
+}
+
+teardown() {
+  global_teardown
+}
+
+@test "(version) version, -v, --version" {
+  run /bin/bash -c "dokku version"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku -v"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku --version"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+}