slapt-src: Handle --config=FILE option properly

This is a followup to commit 3a65be4a18.

completions/slapt-src

@@ -44,6 +44,10 @@ _slapt_src()
             __expand_tilde_by_ref config
             break
         fi
+        if [[ ${words[i]} == --config=?* ]]; then
+            config="${words[i]#*=}"
+            break
+        fi
     done
     [[ -r "$config" ]] || return