Custom tags

Custom tags. Add them to config.ld like this:

    custom_tags = {
      { 'path', title = 'Resource path',
        format = function(v) return '<pre>' .. v .. '</pre>' end } ,
    } 

This is a quick way to get ldoc to recognize arbitrary tags and document them just after the item description. All tags are registered as multiline for now, but this could be improved later.

`custom_tags` is a numeric table containing tables with keys:
- `1`: The tag name
- `title` (optional): A label for the h3, like "Parameters"
- `format` (optional): A function to format the tag's value 

If you don't need title or format you can use string values instead of tables:

    custom_tags = {  'sometag', 'anothertag', }