Add footer function!

[cansavvy]

I added a function that in general adds a given line to the end of each file (but before the references).

Here's how it works:

survey_line <- "*Please provide any feedback with [this form!](https://forms.gle/hc8Xt3Y2Znjb6M4Y7) We appreciate your thoughts.*"

leanbuild::bookdown_to_leanpub(footer_text = survey_line)

And it works!

I will separately in our leanpub course template repository file a PR that puts this into action in our own course templates.