Adding entity support.

@jdorfman thoughts?

From usage:
```
  Entities
  ----------

  A limited implementation of browser HTML Entities.

  $(entity spades) $(entity clubs) $(entity hearts) $(entity diams)

  ♠ ♣ ♥ ♦

  Entities can also be referenced via 'icon':

  $(icon spades) $(icon clubs) $(icon hearts) $(icon diams)

  ♠ ♣ ♥ ♦

  Argument list:

    sbquo:  ‚        nbsp:         dagger:  †        quot:  "   
     larr:  ←         amp:  &        euro:  €      rasquo:  ›   
    frasl:  ⁄        dArr:  ⇓       ndash:  –      cararr:  ↵   
   bullet:  •          gt:  >       clubs:  ♣        rArr:  ⇒   
      sum:  ∑        hArr:  ⇔        uarr:  ↑       rdquo:  ”   
    rsquo:  ’        lArr:  ⇐       diams:  ♦      lsaquo:  ‹   
    empty:  ∅        darr:  ↓       ldquo:  “      hearts:  ♥   
       lt:  <      spades:  ♠       lsquo:  ‘       oline:  ‾   
     rarr:  →       minus:  −        harr:  ↔       mdash:  —   
       pi:  π        uArr:  ⇑    



```