Support plugin pinning

New supported option called 'pinned' for the Plugin command. When set to
1, the plugin is added to rtp but no install/upgrade operation is
performed.

It not only allows vundle to do rtp management of plugins on VCS other
than git, it also allows leaving plugins that had previously been
managed by vundle in the current state, with no further updates.

Fixes #24, #397

autoload/vundle.vim

@@ -54,6 +54,7 @@ sign define Vu_new      text=+  texthl=Comment
 sign define Vu_updated  text=*  texthl=Comment
 sign define Vu_deleted  text=-  texthl=Comment
 sign define Vu_helptags text=*  texthl=Comment
+sign define Vu_pinned   text==  texthl=Comment
 endif