Modularize styling of arrows

[epatters]

We currently have an atomic concept of "arrow style." As seen in #322, this is becoming duplicative. We should refactor the styling of arrows so that the following aspects of an arrow can be styled independently:

• Line style: single line, double lines, etc
• Arrow head
• Arrow tail
• Head decoration, e.g., plus or minus sign
• Tail decoration
• Midpoint decoration, e.g, vertical bar

While conceptually straightforward, it will take some work to ensure that all these options play nice together in both HTML/CSS and SVG rendering.