Add ability to delete multiple buffers

[Otard95]

Initial tests seems promising.

The call signature is the same for the original function, as to not to add any breaking changes. The new function simply takes the same parameters and passes them along, except it splits the last containing buffer name(s), and calls the original for each.

[vds2212]

I have created a Pull request https://github.com/moll/vim-bbye/pull/21 that support:

Multiple arguments: :Bdelete 2 4 5
Ranges: :%Bdelete, :2,4Bdelete or :2Bdelete