Add {imagesprefix} to env-github to display images in github

[prudhomm]

in most examples images are not shown in github because the path information is not known to github

to fix this we need to

1. add imagesprefix definition at toplevel of .adoc file
2. use {imagesprefix} in image::{imagesprefix}... (Note that there must be no / after {imagesprefix} otherwise it will break antora build

see #139 for an example