Fix skiplist matching on Windows

See issue #52

In order to enable platform-agnostic patterns in `g:startify_skiplist`, as long as `shellslash` is set in a user's `.vimrc` before loading the plugin, patterns in `g:startify_skiplist` will work even if they use forward slashes.
Merge pull request #81 from ZeroKnight/patch-1

Fix skiplist matching on Windows