Add argument to select output's class

[meirelesff]

Instead of returning a tibble by default, we can actually give users the chance to choose among the following options:

• tibble (can be the default because of its nice printing);
• data.frame
• data.table

We might call this argument return (i.e., return = c("tibble", "data.frame", "data.table"). Example:

library(siconvr)

progs <- get_siconv("programas", return = "data.table")