Restore `B` command.

Commit 3c0b11f3dd2eb05bb19b4db8e078fe96ff015e61 (2017-09-18) removed the
`B` key that permitted toggling `g:bufExplorerOnlyOneTab`.  Restore this
command to bring back this documented functionality.

plugin/bufexplorer.vim

@@ -514,6 +514,7 @@ function! s:MapKeys()
     nnoremap <script> <silent> <nowait> <buffer> <s-cr>        :call <SID>SelectBuffer("tab")<CR>
     nnoremap <script> <silent> <nowait> <buffer> a             :call <SID>ToggleFindActive()<CR>
     nnoremap <script> <silent> <nowait> <buffer> b             :call <SID>SelectBuffer("ask")<CR>
+    nnoremap <script> <silent> <nowait> <buffer> B             :call <SID>ToggleOnlyOneTab()<CR>
     nnoremap <script> <silent> <nowait> <buffer> d             :call <SID>RemoveBuffer("delete")<CR>
     xnoremap <script> <silent> <nowait> <buffer> d             :call <SID>RemoveBuffer("delete")<CR>
     nnoremap <script> <silent> <nowait> <buffer> D             :call <SID>RemoveBuffer("wipe")<CR>