make biab branch-aware

[gregcaporaso]

Currently biab html and biab ipynb assume that the branch should be master when generating links. It'd be nice to have a command line parameter for this that defaults to master, but can be overwritten for testing. This is useful when working on big PRs that involved a lot of work in a non-master branch (for example https://github.com/gregcaporaso/An-Introduction-To-Applied-Bioinformatics/pull/140).

Related to #5.