fix BundleInstall! on windows

- closes #45
- add /d swithc to cd command to perform drive change
- eclose joined commands with quotes

autoload/vundle/installer.vim

@@ -66,6 +66,11 @@ func! s:sync(bang, bundle) abort
   if isdirectory(git_dir)
     if !(a:bang) | return 0 | endif
     let cmd = 'cd '.shellescape(a:bundle.path()).' && git pull'
+
+    if (has('win32') || has('win64'))
+      let cmd = substitute(cmd, '^cd ','cd /d ','')  " add /d switch to change drives
+      let cmd = '"'.cmd.'"'                          " enclose in quotes
+    endif
   else
     let cmd = 'git clone '.a:bundle.uri.' '.shellescape(a:bundle.path())
   endif