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

test/vimrc

@@ -58,6 +58,13 @@ Bundle 'vim-scripts/RubySinatra'
 " syntax issue #203
 Bundle 'jimenezrick/vimerl'
 
+" Static bundle: Same name as a valid vim-scripts bundle
+Bundle 'latte', {'pinned' : 1}
+if !isdirectory(expand(bundle_dir) . '/latte')
+  call mkdir(expand(bundle_dir) . '/latte', 'p')
+endif
+
+
 filetype plugin indent on      " Automatically detect file types.
 
 set wildignore+=doc            " should not break helptags