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>

ui-shared.c

@@ -864,7 +864,7 @@ void cgit_print_pageheader(struct cgit_context *ctx)
 		if (ctx->repo->max_stats)
 			cgit_stats_link("stats", NULL, hc(ctx, "stats"),
 					ctx->qry.head, ctx->qry.vpath);
-		if (ctx->repo->readme)
+		if (ctx->repo->readme.nr)
 			reporevlink("about", "about", NULL,
 				    hc(ctx, "about"), ctx->qry.head, NULL,
 				    NULL);