Support rotating the layouts

Brought up by @kwcantrell in https://github.com/kwcantrell/empress/pull/49#issuecomment-674294457 and https://github.com/kwcantrell/empress/pull/49#issuecomment-674299580.

This would be really nice for the rectangular layout (since it'd let users draw the tree going from top to bottom, which is a common way of representing them) and for the circular layout (if the user wants the "start point" of the circle to be at an angle other than 0 radians).

This might break the "maximum displacement" computations for rectangular barplots a bit, but fixing it _probably_ won't be too bad. (It might be easier to just support increments of 90 degrees for the rectangular layouts; not sure right now.)

Start by tracing how the rectangular and circular layouts are rendered and where rectangular barplot maximum-displacement computations are performed. Done means users can rotate both layouts, including top-to-bottom rectangular trees and a configurable circular start angle, without breaking those computations.

Written by the indexing model from the issue text.