Add Vector Plots

- The vectors can be plotted via a combination of a vector-field function
  and a grid, or by providing a list of pre-computed values.

- Heads and tails' styles are configurable. As for shapes, for now the
  only option for heads is a triangle, implemented via a polygon. Backends
  may provide more shapes, e.g. Diagrams have fancy ones.

- The support might be a good foundation for Stream Plots as well.
Merge pull request #46 from enomsg/master

Add Vector Plots