Allow biteydown to be run from anywhere / allow user-supplied CSS

In the current HEAD e259ae7a4eb643c3f98c998a7bb01ec73f5dccd5, the css path is hardcoded to "style/style.css"; this is based on the user's current directory. This prevents biteydown from being run except from the biteydown directory, unless the user happens to have a "style/style.css" file in their current directory.

Use Case:
I have a repo for my CV, with biteydown as a submodule.
I'd like to run biteydown from the root of the repo, rather than CD into biteydown every time.

Commit 68fdace fixes this defect.
Commit c203884 builds on 68fdace and allows for the user to specify the path to their preferred CSS, rather than use the CSS vendored with biteydown.

The other commits update the spec and the README as well.