Add newlines command

[mfontanini]

This adds a new_lines (newlines also works) and a new_line (newline also works) command that lets you add line breaks explicitly in a slide. e.g.

hi

<!-- new_lines: 10 -->

mom

<!-- newline -->

hi

Will add an extra 10 new lines in between the first 2 lines of text, and an extra one in between the second and third.