Add ability to specify custom field types and simplify macro usage

Right now when you define a field you can also describe its label using the "fieldNames" macro parameters. However you're forced to define labels for all fields even if you wish to use default label names (which you get only if you don't have any custom field). In addition there's no current solution for defining type.

So the idea is to introduce a new notation and to deprecate fieldNames.

New notation example:

{{jira id="xwikiorg" fields="'Date of First Response'!date, key:"My Key", note, 'Custom Link'!url" fieldNames="'Date of First Response', 'The key to the issue', 'Some Note'"}}
XWIKI-1000|Note
XWIKI-1001
{{/jira}}

In this example:

• "Date of First Response" is a custom field and it's defined of type "date" (which means the custom field content will be parsed as a date and displayed as a date)
• "key" has a label of "My Key" specified
• "Custom Link" is a custom field defined as a "url" type. Its value will be displayed as a link