Add `slide` structure

The basic structural element of a presentation is a slide. Amongst the various
ways to create a slide environment, I define slides as a _section heading_ and
make it borrow all the default elements from `subject` heading.
Tweak slide style

Set the slide style according to your preference. I use the following setup:

- font: Sans Serif Bold font at 1.72 times the default bodysize (the `c` at the
  end of `\ssbfc`)
- color: darkred
- alignment: middle aligned slide title

Read:

- http://wiki.contextgarden.net/Titles
- http://wiki.contextgarden.net/Command/setupheads

for details on how to configure section heads.
Start a new page for each slide

Adding `page=yes` ensures that each slide starts a new page. If a slide
contains more material, it will overflow one page (and no title will appear on
the next page). So, it is the responsibility of the user to ensure that a slide
has the right amount of text.