Map multiple key sequences to the same callback

#### Description

Can we add a "many-to-one" behavior to the key mappings, so for instance if my `.vimrc` says

        let g:NERDTreeMapHelp = ['<F1>', '?', 'h']

I would have 3 ways to open the help menu?


And in the help menu itself we would see

        " <F1>|?|h : toggle help

Thank you!
