Add apple touch icon (#381)

* Add \\( and \\[ as mathjax delimiters

* Make it possible to enable katex and mathjax globally in config.toml

* Add apple-touch-icon

* Add default apple touch icon

Co-authored-by: KK <bebound@users.noreply.github.com>

layouts/_default/baseof.html

@@ -83,6 +83,9 @@
     <link rel="icon" type="image/png" href="{{ .Site.Params.favicon_32 | default "/images/favicon-32x32.png" | absURL }}" sizes="32x32">
     <link rel="icon" type="image/png" href="{{ .Site.Params.favicon_16 | default "/images/favicon-16x16.png" | absURL }}" sizes="16x16">
 
+    <link rel="apple-touch-icon" href="{{ .Site.Params.touchicon | default "/images/apple-touch-icon.png" | absURL }}">
+    <link rel="apple-touch-icon"  sizes="180x180" href="{{ .Site.Params.touchicon | default "/images/apple-touch-icon.png" | absURL }}">
+
     {{ range .AlternativeOutputFormats -}}
       {{ printf `<link rel="%s" type="%s" href="%s" title="%s" />` .Rel .MediaType.Type .Permalink $.Site.Title | safeHTML }}
     {{ end -}}