Add filter to exclude metabox for a custom post type

[hofmannsven]

Adds the ability to define a custom filter to exclude the plugin metabox for a custom post type.

This change improves the compatibility with other plugins like SplitMagic, Custom Field Suite or Advanced Custom Fields (see issue #9) that make use of custom post types without support for an editor.

Usage within functions.php:

add_filter( 'bsf_exclude_custom_post_type', 'exclude_splitmagic_custom_post_type' );
function exclude_splitmagic_custom_post_type() {

    return array( 'splitmagic_test' );

}

[Nikschavan]

Thank you very much for this @hofmannsven, Although the correct solution for #9 will be the PR #50 which actually fixes this problem.

I will merge this filter as well as this will be beneficial for folks who want to disable the metabox completely on a selected post types.