Do not clobber existing mappings.

]e and ]b are already used by https://github.com/tpope/vim-unimpaired which is a very popular plugin.

By providing `<Plug>` mappings, users can use the default convenience mappings if they want to, or ignore them without having to explicitly undo diffchar's mappings. Example vimrc setting:

    nmap ]b <Plug>(DiffChar_ToggleAllLines)