Add function ctrlp#clearmarkedlist()

I have [a plugin](https://github.com/d11wtq/ctrlp_bdelete.vim) that integrates with ctrlp to allow buffers to be marked and then deleted.

Currently there's [a bug](https://github.com/d11wtq/ctrlp_bdelete.vim/issues/2) in my plugin that cannot be fixed on my side alone, since it requires "resetting" or "emptying" ctrlp's internal `s:marked` dictionary.

This simply defines a function that can be called to clear the marked list.

If you know another way I can achieve this without adding such a function to ctrlp, I'd be happy to do that instead.