Add function to create codebook based on XLSForm

[ernestguevarra]

Given an existing XLSForm, create appropriate codebook to match all variables created by form.

[jwrozelle]

I've developed a rudimentary version of this on figured.io. Curious what your current vision are, but probably wouldn't be too difficult to integrate that code into the odkr package. The Nafundi codebook generator doesn't seem to play nice with current versions of odk.