New syntax for vertical slides

This release completely changes the syntax for vertical slides in favour of a more elegant syntax that is consistent with the rest of the API. The new syntax is:

@show.slide
def main_slide():
    # content

    @show.slide
    def vertical_1():
        # content

    @show.slide
    def vertical_2():
        # content

TL;DR: vertical slides are now created by declaring a function with the @slide decorator inside the main slide to which they are vertical.

Thanks to @tialpoy for the suggestions.