DokuWiki writing now implemented

DokuWiki writing now implemented - this is the writing half of #386

All the items listed in https://github.com/jgm/pandoc/issues/386#issuecomment-47491034 are now addressed, and a number of other improvements besides. (It turned out that superscript and subscript were already behaving correctly)

I'll log a few separate issues, soon, to summarise the known outstanding issues, but I feel this is basically robust enough to be worth wider testing now.