Ensure certificate directory exists

Before choosing a permanent client certificate, Elpher tries to list
existing certificates. This fails if `elpher-certificate-directory` does
not exist, e.g.

```
---- ERROR -----

When attempting to retrieve gemini://alexschroeder.ch/do/comment/2020-08-14_How_to_comment_using_Elpher:
Opening directory: No such file or directory, /home/michel/.emacs.d/elpher-certificates

----------------

Press 'u' to return to the previous page.
```

Fix by creating the directory unless it exists in
`elpher-list-existing-certificates`.

Signed-off-by: Michel Alexandre Salim <michel@michel-slm.name>