Composition functions and operators

[ijlyttle]

First proposals:

vlc_hconcat <- function(spec, ...), infix operator: |

vlc_vconcat <- function(spec, ...), infix operators: &, /

vlc_concat <- function(spec, ..., columns = 2), infix operator: ~

vlc_colunms <- function(spec, columns = 2)

The vlc_columns() function would act on a spec from vlc_concat() to define the number of columns.