Fix tempfile suffix logic

[samuelallan72]

Previously the file extension (suffix) only depended on
the value of cfg_default_markdown,
which wasn't helpful for editing existing notes.

Ensure that for existing notes,
the file extension for tempfiles passed to the editor
matches that of the note type (plain text or markdown).
Retain the old behaviour for new notes.

[pataquets]

Tested OK.
Before PR: temp filename (as reported by $EDITOR) has txt extension for MD notes.
After PR: temp filename (as reported by $EDITOR) has mkd extension for MD notes.
Also tested for non-MD notes. No change on behaviour.
Partly solves #120.
LGTM.