Adds a print button to the top of estimates and invoices.

functions.php

<?php

/**
 * Adds a print button to the top of estimates and invoices.
 * @return type
 */
function si_print_button() {
	?>
		<a href="javascript:window.print()" id="print_button" class="button print_button"><?php si_e('Imprimir') ?></a>
	<?php
}
add_action( 'si_doc_actions_pre', 'si_print_button' );