config: add url.uri-characters

This option specifies the characters allowed in the auto-detected
URLs.

Any character not in this set constitutes an URL delimiter, and will
never be included in auto-detected URLs.

This option does not affect OSC-8 URLs.

Closes #654

config.h

@@ -151,6 +151,7 @@ struct config {
         } osc8_underline;
 
         wchar_t **protocols;
+        wchar_t *uri_characters;
         size_t prot_count;
         size_t max_prot_len;
     } url;