New filters & a new function for customising tax display on the Cart page

When the **Display Tax Totals** setting is set to **As a single total** (i.e. http://cl.ly/image/3P2Z1D0T0S28) there is no way to hook into to the tax total to filter the output displayed on the cart page (`cart-totals.php`).

That means Subscriptions cannot display the correct tax total string. e.g. it should look like this: http://cl.ly/image/0L1i16170U34 but instead looks like this: http://cl.ly/image/3g2s210O1C3e.

This pull request fixes that by wrapping the call to `wc_price( WC()->cart->get_taxes_total() )` in a new `wc_cart_totals_taxes_total_html() ` function and passing it through a new `'woocommerce_cart_totals_taxes_total_html'` filter.

It also introduces a related `'woocommerce_cart_taxes_total'` filter for customising the actual total.

Relates to ZD ticket 160027.