add key mapping for c-d
unload buffer on c-d
restore position of selected item
unload buffer on c-d in buffer-explorer
Merge branch 'implement-unload-buffer' of https://github.com/alovak/lusty into alovak-implement-unload-buffer

Conflicts:
	plugin/lusty-explorer.vim
	src/lusty/buffer-explorer.rb

Also, remove unload_selected_buffer stuff from explorer.rb since it's
now in buffer-explorer.rb.