allow additional arguments to be passed to stats::stl()

[RoelVerbelen]

time_decompose() accepts additional parameters ... which are being passed to the underlying functions decompose_stl() and decompose_twitter(). However these functions currently don't have ellipsis. This PR suggests to allow additional parameters to be passed to the underlying function call to stats::stl(). That would allow modifying the default STL decomposition, e.g.:

library(anomalize)
library(dplyr)

data(tidyverse_cran_downloads)

# Basic Usage
tidyverse_cran_downloads %>%
    time_decompose(count, method = "stl", robust = FALSE, t.degree = 0)