Add a shortcut for selecting multiple annotations

[karoliskoncevicius]

Right now selecting multiple annotations from row or column metadata can only be done via colanns():

library(annmatrix)
example(annmatrix)  # to get a workable X annmatrix object for demo

colanns(X, c("id", "name"))

However, it would be convenient to access the same using a shortcut, like with X$name. One idea is to do this via a string:

X$'id,name'