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/config.vim

@@ -134,3 +134,7 @@ func! s:bundle.path()
   return s:expand_path(g:bundle_dir.'/'.self.name)
 endf
 
+func! s:bundle.is_pinned()
+  return get(self, 'pinned')
+endf
+

autoload/vundle/installer.vim

@@ -67,6 +67,8 @@ func! vundle#installer#run(func_name, name, ...) abort
     echo n.' deleted'
   elseif 'helptags' == status
     echo n.' regenerated'
+  elseif 'pinned' == status
+    echo n.' pinned'
   elseif 'error' == status
     echohl Error
     echo 'Error processing '.n
@@ -219,6 +221,11 @@ func! s:helptags(rtp) abort
 endf
 
 func! s:sync(bang, bundle) abort
+  " Do not sync if this bundle is pinned
+  if a:bundle.is_pinned()
+    return 'pinned'
+  endif
+
   let git_dir = expand(a:bundle.path().'/.git/', 1)
   if isdirectory(git_dir) || filereadable(expand(a:bundle.path().'/.git', 1))
     if !(a:bang) | return 'todate' | endif