refactor(item): Create .icon-accessory and remove auto nav icons, closes #1061

BREAKING CHANGE: The developer should be stating exactly how an icon
should show, but previously the right nav arrow icon violates this by
automatically showing a right arrow when an item was an anchor or
button. Instead of using the `:after` item selector, which was always
applied by default, it uses the same markup as `item-icon-right`, which
is easier to understand, customizable and not a hard coded default.

This change removes the `:after` nav icon styling, and creates a new
class, `icon-accessory`, based off of similar CSS. The change makes a
nav arrow highly customizable, allows RTL developers to easily control
the arrow direction, and the accessory class is something that's
reusable.

An example of right side arrow using `ion-chevron-right` as the icon:

    <a class="item item-icon-right" href="#">
      Check mail
      <i class="icon ion-chevron-right icon-accessory"></i>
    </a>