Added apply_filters to edd_purchase_variable_pricing

Added apply_filters to edd_purchase_variable_pricing() function tp allow extra content to be added after each variable priced download. (https://easydigitaldownloads.com/support/topic/plugin-variable-priced-download-descriptions/#post-36310)

This would allow something like the following, so I don't have to replicate the entire function. Thoughts?

    function edd_vpd_output_description( $content, $price ) {
	    $content = '<p class="edd-variable-pricing-desc">' . esc_html( $price['description'] ) . '</p>';
	    return $content;
    }
    add_filter( 'edd_after_price_option', 'edd_vpd_output_description', 10, 2);

Also, I'm not sure if i'm correct in doing so, but I changed that last `add_action( 'edd_after_price_options', $download_id );` to `do_action( 'edd_after_price_options', $download_id );` to match the first do_action( 'edd_before_price_options', $download_id );` you have inside that function.