Add Crayon Syntax Highlighter support to the 'remote-markdown' shortcode

Hi Andrés,

as I am using the [Crayon Syntax Highlighter](https://wordpress.org/plugins/crayon-syntax-highlighter) plugin in my Wordpress instance, I thought I could add the support for it to your wp-remote-markdown plugin.

Could you please have a look at it and maybe merge it to the master branch and release a new version? (If you like this small feature).

Regards,

Guy