Add support for character entities in XWiki Syntax

I would imagine something like:

char:&entity_name;
char:&#entity_number;

I would use it to specify things like

[[char:—]]

to achieve '—', instead of writing

{{html}}—{{/html}}

Other popular entities:

• Space

   

• Ampersand &

  & &

• Arrow ▾

  ▾

• Section §

  § §

• Euro €

  € €

• Registered ®

  ® ®

• etc.

Character entity references are used normally for typographic use cases or instead of using an image replacement.

See more entities:
http://www.w3.org/wiki/Common_HTML_entities_used_for_typography
http://www.alanwood.net/demos/ent4_frame.html
http://www.unicode.org/charts/