Add mirroring for plugins aswell

Hi there,

just saw your awesome changes. Sadly the jenkins-ci.org mirror were so slow today and earlier this week and didn't even worked for hours. So I've just implemented the possibility to switch the attribute mirror for plugins.

This does only work, if the mirror parameter is transmitted to the plugin lib. I've implemented it into the plugins recipe by default.

Also I had to make some changes to your attribute, I wrote everything down in the README.

Enjoy.
Hendrik