Add ability to number headers

there are 2 levels where this could be useful:

• level of the full wiki. It could be wiki configuration property. Not sure this would be used a lot so maybe not needed
• level of the document. This can be done using semantic properties, see XWIKI-3320

Example:

%header-style numbered
= A
== B
= C

would display:

1 A
1.1 B
2 C