(chise-wiki-displayed-features): New variable.

(chise-wiki-displayed-features): New variable.
(char-feature-name-at-domain): Fix problem when `domain' is nil.
(char-feature-name-domain): New function.
(www-uri-make-feature-name-url): New function.
(www-format-props-to-string): Fix problem when :len's value is string.
(www-format-eval-unit):
- When :feature is specified, prefer FEATURE@DOMAIN to FEATURE.
- When :feature is specified in value related format functions, add
  the displayed feature-name to `chise-wiki-displayed-features'.
- Support `name-url' and `domain-name'.