Add tidiers for stats::decompose and stats::stl

[atheriel]

This PR adds an augment s3 method for R's two built-in seasonal decompositions. It does not add tidy or glance support, since neither of these methods produce model output, per se.

The augment method struck me as a good for turning the output of these decompositions into something that can be added back to the data. So, for example (using the datasets::nottem time series):

# Perform seasonal decomposition on the data with both decompose and stl:
d1 <- stats::decompose(nottem)
d2 <- stats::stl(nottem, s.window = "periodic", robust = TRUE)

# Compare the original series to these two decompositions.
cbind(broom::tidy(nottem), broom::augment(d1), broom::augment(d2))

The Roxygen documentation contains the above example (demonstrating basic usage) and a much longer one for visualising both types of decomposition using ggplot2. I don't know what the policy is on including these larger examples, but I'm happy to amend this one to fit the style of the project.

Similarly, I don't know what the policy is on adding test for new tidiers, but again I'm happy to write them.

[dgrtwo]

Thanks!

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