Allow overriding bundle name

This changeset allows the second parameter of the Bundle command (a
dictionary) to override bundle object fields that come from parsing the
bundle spec. This means that
 Bundle 'abolish.vim', {'name' : 'abo'}
will be installed in a bundle directory named 'abo', rather than
'abolish.vim', which is the name parsed from the bundle spec.

autoload/vundle/config.vim

@@ -38,7 +38,7 @@ func! vundle#config#init_bundle(spec, opts)
 
   " Combine info derived from the spec with the options from the Bundle
   " command.
-  let opts = extend(s:parse_options(a:opts), s:parse_spec(spec))
+  let opts = extend(s:parse_options(a:opts), s:parse_spec(spec), 'keep')
 
   " Include generic bundle methods
   return extend(opts, copy(s:bundle))