Merge add-after-merge-plugin-hook (#129) into master

Plugins: create after_merge invocation stage

After merging, we want to remove certain labels... this makes that possible

bin/feature

@@ -215,6 +215,8 @@ when 'merge'
       "git branch -d \"#{feature}\""
    ])
 
+   Plugins.invoke :after_merge, :feature, feature
+
    puts
    puts "Successfully merged feature-branch: #{feature} into #{dev_branch}"
    puts "If you are satisfied with the result, do this:\n" + <<CMDS

bin/hotfix

@@ -204,6 +204,8 @@ when 'merge'
       "git checkout #{stable_branch}"
    ])
 
+   Plugins.invoke :after_merge, :hotfix, hotfix
+
    puts "Successfully merged hotfix branch: #{hotfix} into #{stable_branch} and #{dev_branch}"
    puts "If you are satisfied with the result, do this:\n" + <<CMDS
       git push