x/tools/cmd/present: support sub-sections in slides

[griesemer]

For the following input (bug.slide):

Title

* Subtitle

** foo
- abc

** bar
- abc

the 2nd page looks like this:

That is, the subtitles are printed on top of each other. Generally, they don't seem to do what would be expected.

This is on tip, using Chrome (Version 42.0.2311.152 (64-bit)) on OS X Yosemite (Version 10.10.3 (14D136)).

[adg]

Subtitles aren't supported in the slide output of present. They're for the blog and other mediums.

[adg]

Dupe of #6846