add offset.label and label to geom_treescale

[xiangpin]

add offset.label and label to geom_treescale, these parameters will control the title of tree scale. offset.label represents the distance between line and title of tree scale. #359

for example

library(ggtree)
library(ggplot2)
library(patchwork)

set.seed(2020)
x = rtree(5)

p1 <- ggtree(x) + geom_treescale(x=2, y=1, offset=.1, offset.label=-0.2, label="substitution rate")

p2 <- ggtree(x) + geom_treescale(x=2, y=1, offset=-.2, offset.label=0.1,label="substitution rate")

p <- p1 + p2
p

I had tried convert x and y to npc unit, and the width should be keeped native unit. But it can not create data.frame to plot tree scale. I think if we want to convert x and y to npc unit, we might need to create treescaleGrob.

[GuangchuangYu]

the unit parameter is not in high priority. Let's postpone to do it.