add support for building image links on the fly

[gregcaporaso]

in IAB, I hard code image links as:

https://raw.githubusercontent.com/gregcaporaso/An-Introduction-To-Applied-Bioinformatics/master/book/fundamentals/images/upgma-tree-final.png

It'd be nice to auto-generate the part of those links that never change (which could also support branch-awareness by replacing master with a branch name): https://raw.githubusercontent.com/gregcaporaso/An-Introduction-To-Applied-Bioinformatics/master/book/