Added plugin to update CHANGELOG.md on bump

[allanlang]

Our team would like to ensure the CHANGELOG.md is kept in sync with version bumps applied to metadata.rb, and since we often (for various reasons) need to bump a version with no other meaningful changes, I wrote this simple plugin to do just that. We can use it through the custom plugin mechanism but thought I would submit it for general consideration.

[jonlives]

Thanks for this! Added to next release milestone.

[allanlang]

You're very welcome, thanks for creating a neat tool!

[chazzly]

This was the original intent behind #174. I went the extra step of trying to pass the comment on as a commit message and ran into problems passing the options from the bump plug-in to the git plug-in.

[lizgene]

@allanlang or @jonlives, I have a pretty simple question for you. I was just looking through issues about plugins and found this one. I'm wondering if one of you can point me to an example or documentation on how to include a custom plugin? I created one in my chef directory under /plugins/chef/spork/my_custom_plugin.rb and followed the format described in the plugin README (I have require 'knife-spork/plugins/plugin and define my module/class in the same structure as the official plugins)

Still, running a knife spork info doesn't show my custom plugin