Improve the Previous and Next button markup

[jasonwebb]

• Move the icon to an inner DOM element so that it can be hidden from screen readers using aria-hidden="true".
• Wrap the inner text in a <span> and apply CSS to make it visible to screen readers only.
• Remove the aria-label once the above changes are made, since the inner text will act as a more robust accessible name (see the First Rule of ARIA Use).
• Document changes in the main README

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Currently the buttons render like so:

<button class="slick-prev slick-arrow" aria-label="Previous" type="button">
  :before
  Previous
</button>

Where an unrecognizable (to screen readers) custom font character is injected via the :before pseudo-class.

What we really want is this:

<button class="slick-prev slick-arrow" type="button">
  <span class="slick-previous-icon" aria-hidden="true"></span>
  <span class="slick-sr-only">Previous</span>
</button>

For reference, see this nice section about icon buttons in the MDN's documentation for the <button> element.