Add commands for deleting all buffers and deleting all but one buffer

[qiushihe]

Hi,

I added two commands:

• Bdeleteall which deletes all buffers; and ...
• Bdeleteexcept which deletes all but one buffer (defaults to the current buffer)

I've been using this for a while now and it seems fairly stable. So please consider merging this :D

Thanks!

[alexanderjeurissen]

@qiushihe @moll I don't think there is use for the Bdeleteall command seeing as that can already be achieved using :bufdo :Bdelete (as specified in the readme) and deleting all buffers that are currently hidden can be achieved with (https://github.com/arithran/vim-delete-hidden-buffers)

[moll]

Thanks, @qiushihe, for sharing your code! I'm not yet sure if this should belong in Bbye.vim, but I've mentioned your script in the README to at least people with similar wishes can still copy it over to their config files. :)

[croobat]

Hey, sorry I don't know if I can PR a closed PR but at least can leave a comment for future reference, for the function to work you gotta add "bdelete" to the bdelete function arguments called around line 21, as in:

call s:bdelete("bdelete", a:bang, buffer_nr)