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

doc/vundle.txt

@@ -166,6 +166,26 @@ This can be used to prevent name collisions between plugins that Vundle would
 otherwise try to clone into the same directory. It also provides an additional
 level of customisation.
 
+The 'pinned' option
+-------------------
+
+A flag that, when set to a value of 1, tells Vundle not to perform any git
+operations on the plugin, while still adding the existing plugin under the
+`bundles` directories to the |runtimepath|.
+
+For example:
+>
+  Plugin 'mylocalplugin', {'pinned': 1}
+
+This allows the users to include, with Vundle, plugins tracked with version
+control systems other than git, but the user is responsible for cloning and
+keeping up to date. It also allows the users to stay in the current version of
+a plugin that might have previously been updated by Vundle.
+
+Please note that the URI will be treated the same as for any other plugins, so
+only the last part of it will be added to the |runtimepath|. The user is
+advised to use this flag only with single word URIs to avoid confusion.
+
 3.2 SUPPORTED URIS ~
                                                         *vundle-plugins-uris*
 
@@ -185,7 +205,7 @@ GitHub is used when a user/repo is passed to `Plugin`.
 
 Vim Scripts
 -----------
-Any single word without a '/' is assumed to be from Vim Scripts.
+Any single word without a slash '/' is assumed to be from Vim Scripts.
 >
   Plugin 'ctrlp.vim' => https://github.com/vim-scripts/ctrlp.vim