Add toggle to use <button> instead of <input type="button">

[nielslange]

Fixes #90

In https://wordpress.org/support/topic/template-override-not-working-8/, a user asked how to overwrite the template respectively how to change <input type="button"> to <button>. This PR aims to a toggle to use <button> instead of <input type="button">.

Testing instructions:

• Install and activate the Code Snippets plugin.
• Add PHP filter add_filter( 'use_html_buttons', '__return_true' );.
• Go to a product page and verify that the button elements instead of input elements are visible.
• Also verify that the quantity can still be changed.
• Add the product to the cart and repeat the previous two steps.