add post type selection for custom post types

Hi,

As requested in -> https://meta.discourse.org/t/custom-fields-and-custom-post-types-ex-article-recipe/7116 and also -> https://meta.discourse.org/t/wp-discource-not-working-on-custom-post-types/15469, I've added the ability to select custom post types (along with the default 'Post' & 'Page').

Have also added filtering so that the 'Publish to Discourse' checkbox does not show up on unselected post types (to avoid confusion).

Cheers