Wiki pragma directives

[cboos]

We could add pragma directives for the Wiki engine.

This could be used to modify the way the engine behaves, as the pragmas are seen.

Use Cases

• Section Numbering (similar to the reST's .. sectnum:: directive)
  

• Respect line breaks (we currently do that programmatically)
  

Implementation

There could either be a specific syntax for this
(e.g. like MoinMoin ​pragmas), or a [[Pragma]]` macro.

The macro could be a special macro, like [[BR]] is, or a regular macro
once the formatter object will be given to macros.

The macro will simply set a formatter "user property",
e.g.

  formatter[pragma_name] = pragma_value

In other parts of the engine, such properties could be explicitely retrieved
e.g. in the _heading_formatter:

   do_numbering = formatter['sectnum']

[cboos]

WikiEngine refactoring and related fixes postponed.

[cboos]

The suggested MoinMoin syntax could be supported if present at the start of a document.
Otherwise, I think a WikiMacro would be more appropriate.