Support Jetpack Markdown out of the box

[BrookeDot]

Currently it's possible to add Jetpack/WordPress.com markdown support by adding the following action in a theme or feature plugin:

add_action('init', 'blicki_markdown_init');
function blicki_markdown_init() {
    add_post_type_support( 'blicki', 'wpcom-markdown' );
} 

This PR allows it to be supported by default when Jetpack Markdown is on as per the documentation here: https://jetpack.com/support/markdown/