add select function to select parts of the pipeline

[rpahl]

The operation should support

• select by position: pip$select(1:14) and pip$select(1, 3, 10)
• select by step names: pip$select("data", "data_rep", "model_fit")
• select by range of step names: pip$select(data:model_fit) or pip$select("data":"model_fit")
• negative selection by position: pip$select(-5, -7) or pip$select(c(-5, -7)) or pip$select(-(5:7))
• negative selection by names: pip$select(-"data", -"data_prep") or pip$select(-c("data", "data_prep")) or pip$select(-("data":"data_prep"))

The select function probably will have two additional args, i.e. the signature will be something like
select <- function(..., .upDeps = TRUE, .downDeps = TRUE)
which by default will select the steps given by the user plus all up- and down-stream dependencies. Then,
if .upDeps == FALSE, the function will throw if upstream dependencies are missing, while
if .downDeps == FALSE it will just provide a message about the dropped downstream dependencies.