add read.mcmctree function

[xiangpin]

Description

a function to parse the output tree of MCMCTree was added.

Related Issue

fix #13

Example

library(treeio)
library(ggtree)
file <- system.file("extdata/MCMCTree", "mcmctree_output.tree", package="treeio")
tr <- read.mcmctree(file)
tr
p <- ggtree(tr)
p + geom_range(range="`0.95`", alpha=0.6, color="blue", size=2)