readme: use string_list instead of space deliminations

Now this is possible in cgitrc -

readme=:README.md
readme=:readme.md
readme=:README.mkd
readme=:readme.mkd
readme=:README.rst
readme=:readme.rst
readme=:README.html
readme=:readme.html
readme=:README.htm
readme=:readme.htm
readme=:README.txt
readme=:readme.txt
readme=:README
readme=:readme
readme=:INSTALL.txt
readme=:install.txt
readme=:INSTALL
readme=:install

Suggested-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cgit.h

@@ -73,7 +73,7 @@ struct cgit_repo {
 	char *owner;
 	char *defbranch;
 	char *module_link;
-	char *readme;
+	struct string_list readme;
 	char *section;
 	char *clone_url;
 	char *logo;
@@ -183,7 +183,7 @@ struct cgit_config {
 	char *mimetype_file;
 	char *module_link;
 	char *project_list;
-	char *readme;
+	struct string_list readme;
 	char *robots;
 	char *root_title;
 	char *root_desc;