Make link reference syntax more generic and extensible

The idea is:

• Have the following canonical form:

  [[label>>referenceType:referenceData]]

• Support the following referenceTypes for the existing types: "doc", "url", "mailto", "attach", "image"
• Support implicit types for "doc" and "url", i.e. autodiscover the type when not specified using the following rule:
  • check if it's a URL by checking for a match on

    *://*

  • if not a URL then consider it's a reference to a document
• Allow new reference types to be plugged as extensions

Note that this should be implemented in XWiki Syntax 2.1 (and not in XWiki Syntax 2.0).