Prevent storing killed buffers in `deft-auto-save-buffers'

I believe this fixes the following issue reported on the EmacsWiki:

> I see some blocking behaviour with defts autosave feature after
> having emacs open for awhile with deft running, any suggestions
> on what this might be (only happens with the autosave in deft)?
> - anon
Use slug instead of full path when calling deft-new-file-named

Fixes a bug introduced in commit c203ee3912be0ccfbc4f4ff9cd78a69dfab89f41,
where creating a file when the filter string and deft-use-filename-as-title
are both non-nil results in an invalid path.
Deft 0.5.1
Documentation: use code backticks for command sequences
Improve docstring for `deft-open-file'