Allow styling of nodes within phylogram.plot()

[ms609]

There are a number of settings where it is useful to be able to style the vertical lines associated with polytomies (example).
I'm hoping that it would be possible to extend the functionality of plot.phylo() to support this.

This PR adds node.color, node.width and node.lty parameters that allow nodes to be styled separately from their pendant edges. If these parameters are unspecified, the existing behaviour of plot.phylo() remains unchanged.

If node.color (etc.) parameters are specified and edge.color is unspecified, then edge colours will be inferred based on the child node.

Example output:

library('TreeTools') # For CollapseNode, BalancedTree
plot(CollapseNode(BalancedTree(10), 18),
     edge.color = viridisLite::plasma(18),
     node.color = c(rep('blue', 10), rev(viridisLite::inferno(9))),
     node.lty = c(rep('solid', 15), rep('dotted', 1), rep('dashed', 2)),
     edge.lty = 'solid',
     node.width = c(rep(1, 10), 1:8),
     edge.width = (1:17) / 2
     )

plot(CollapseNode(BalancedTree(10), 18),
     direction = 'upwards',
     node.color = c(rep('red', 10), rep('grey', 4), rep('red', 2), 'green', 'red'))

I've checked a few basic test cases – but is there a standard set of test plots that I should verify reproduced correctly with the new code?

I have not attempted to extended this functionality to plotting types other than "phylogram".