wikicontent: Use <code> tags instead of indents when converting preformatted text inside of a list or another inline structure.

Change is pretty heavy-handed, involves tracking trailing newlines through the parse tree, but should work in most circumstances.

This is in response to Issue #5, following rules for list items found in https://www.dokuwiki.org/faq:lists#multiline_list_items
yamdwe: Invalidate dokuwiki page cache by touching a config file after the import.

Only necessary if there is an existing Dokuwiki and pages that are already rendered.