enabled checklists' checkboxes for direct interaction

by default, markdown-checklist's checkboxes are disabled to avoid
falsely suggesting interactivity

this proof of concept adds that required interactivity via JavaScript

caveats:
* using TiddlyWeb URIs seems a bit wrong - mostly because this way the
  client has to know about tiddlers' JSON structure
* eventually the JavaScript code should be refactored, with generic
  parts being moved into markdown-checklist:
  https://github.com/FND/markdown-checklist/issues/1
simplified checkbox interactivity

cf. previous commit 80d54377ccd36f02483b66c4ead00f9568a40c49

instead of relying on the tiddler model, we're now making the wiki page
handler either return or accept the respective source as plain text

review from @cdent would be appreciated; not sure whether this new logic
belongs into the `wiki_page` handler (also see inline comments)
refactored client-side checklist augmentation

... splitting it into a generic and an application-specific module -
which (pending documentation) should make it ready to be moved into
markdown-checklist