implement core plugin mgmt using plugn. add tests.

tests/unit/plugin.bats

@@ -0,0 +1,45 @@
+#!/usr/bin/env bats
+
+load test_helper
+TEST_PLUGIN_NAME=smoke-test-plugin
+TEST_PLUGIN_GIT_REPO=https://github.com/dokku/${TEST_PLUGIN_NAME}.git
+
+remove_test_plugin() {
+  rm -rf $PLUGIN_ENABLED_PATH/$TEST_PLUGIN_NAME $PLUGIN_AVAILABLE_PATH/$TEST_PLUGIN_NAME
+}
+
+teardown() {
+  remove_test_plugin || true
+}
+
+@test "(plugin) plugin:install, plugin:disable, plugin:uninstall" {
+  run bash -c "dokku plugin:install $TEST_PLUGIN_GIT_REPO"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku plugin | grep enabled | grep $TEST_PLUGIN_NAME"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku plugin:disable $TEST_PLUGIN_NAME"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku plugin | grep disabled | grep $TEST_PLUGIN_NAME"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku plugin:uninstall $TEST_PLUGIN_NAME"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku plugin | grep $TEST_PLUGIN_NAME"
+  echo "output: "$output
+  echo "status: "$status
+  assert_failure
+}