print.sagelabel only print certain topics

[tlebryk]

In functions like labeltopics, you can filter for specific topics. However, when using sageLabels, you cannot filter and must print information for every topic. This is a small suggestion to change print.sagelabel from:

print.sageLabels <- function(x, ...) {
  topicnums <- 1:x$K

to:

print.sageLabels<- function(x, topics=NULL, ...) {
  if(is.null(topics)) topics<- 1:x$K
  ...

and change topicnums to topics in every subsequent reference.

Happy to take a PR out with this change!