added plugin to get post filename

to create a link to a post, the documentation says it should be {%
post_url andthenthefilename %}
this plugin will get the filename with {{ post.filename }} however, the
post_url isn't working for some reason. Will investigate further later.
added image tags to posts

- added image tags to posts
- used those on the nieuws and index pages to dynamically generate the
pictures.
new class img100 to set an image width to 100%