Comment feature: add plug mapping, conditional map

The mappings for the commenting feature might conflict with plugins like
NERDCommenter. Check if the user disallowed the mapping through a
variable or if it is simply *already* mapped.

Thanks to a <Plug> mapping, the user can also craft his own mapping
without too much trouble.
Comment feature: Document map conflict resolution

Added 'xml_no_comment_map' setting documentation, add a note about
conflict resolution and how to map your own keys.