Allow users to provide a custom displayer in wiki pages

JIRA Macro idea:

{{jira style="custom" parameters="display.custom.template=My.JIRATemplate"}}
...
{{/jira}}

in My.JIRATemplate:

Bindings:
- issues --> List<JIRAIssue>

JIRAIssue extends Element
  - getFieldValue(fieldName)
  - getCustomField(fieldName) --> JIRACustomField

JIRACustomField extends Element
  - getKey()
  - getValue()
  - getName()


- parameters: JIRAParameters object
- displayers (map) : Map of JIRADisplayer
- fields --> List<String> : fields to display

{{velocity}}
#foreach ($issue in $issues)
  #foreach ($field in $fields)
    ## Specific display for a specific custom field
    #set ($customField = $issue.getCustomField($field))
    #if ("$!customField" != '' && $customField.getKey().endsWith(":url"))
      #set ($renderedValue = "[[Lien>>$customField.getValue()]]"
    #else 
      #set ($renderedValue = $displayers.get($field).displayField($field, $issue))
    #end
    * $renderedValue
  #end
#end
{{/velocity}}



Example:
---------

{{jira style="custom" parameters="display.custom.template=My.JIRATemplate"}}
...
{{/jira}}

And in My.JIRATemplate:

{{chart source="inline" ...}}
  #foreach ($issue in $issues)
    ...
  #end
{{/chart}}

vs example 2
-------------

Using directly JRJC API...

TODO: try this first!