add `nextSlice` helper to advance slice to sep and store in a given mutable slice (#214)
add `nextSlice` helper to advance slice to sep and store in a given mutable slice (#214)

* add `nextSlice` helper to advance slice to sep and store in mut arg

This is similar to the `readLine` procedure for the `Stream` type in
the stdlib, except working on slices.

* add note about return value of `nextSlice`
c-blake/cligen