Add hooks for themes to manage adding of numbers

The hooks
1. add_filter("pb_lists_add_numbers_to_list_elements",
function(){return true;});
2. add_filter("pb_lists_add_numbers_to_heading_levels",
function(){return 3;});

allow theme author to 1. add numbering to images, table and headings
(default is false) and 2. to define how many levels of headings get a
number (default is 6).

6 means h1, h2, h3, h4, h5, h6 get numbers. 3 on the other hand would
mean that only h1, h2 and h3 get numbers.