Add C-h synonym for delimitMateBS

When typing in insert mode, one often uses C-h interchangeably with <BS>. This is meant to add C-h support for delimitMateBS.

Tested on MacVim and console vim 7.4. Hopefully I haven't broken anything else.

Note: The same can't be done for delimiteMateS-BS, because vim cannot differentiate between C-h and C-H. Will have to live with that I suppose.