Add advanced slide example

[ryantibs]

We should add a more advanced example of sliding with a function to the slide vignette, very bottom.

Originally this was about sensorization (in the old modeltools vignette), but I think that this example should be in its own vignette, if it exists at all; see #6.

[ryantibs]

@dajmcdon You had an idea, I think it was to implement a simple baseline forecaster (say projecting at the 0.25, 0.5, 0.75 quantile levels)? Can you give it a try on a branch and make a PR? Thanks!

[dajmcdon]

@ryantibs Can you give me push access?