Clean Up Custom Network Layouts

FEATURES

• You can pass a function to your networkType's zoom property that will be passed (nodes, size) where you can change the xy props of the nodes to zoom them the way you want. This is because some layouts don't come back within the bounds of the size you pass (I'm looking at you d3-flextree) and there's no way for Semiotic to guess, so you'll have to handle it.
• showLinePoints used to only take true/false but now honors also "top" and "bottom" which will change the points that appear in an area chart (like stackedarea or bumparea). In the past these used to only show the middle point of the area, but now you can change it to show the top or bottom.