Improved initial (naive) implementation of the relative path navigation.
Pass current document path to the preview window, to allow enable navigation.
Fixed a navigation bug caused by hiding and showing of preview window.
Merge pull request #19 from bigT/local-images-and-navigation

Local images and navigation
Bump to version 2.6.

For updating changes around navigation/relative links from Taras Alenin
and better file extension support from Sam Harwell.