Feature Request: allow for having headers start at H2 or H3 instead of H1

[thomasvs]

use case: I use obsidian which has a built-in present mode that works similarly. I use H1 (#) for the name of the file at the top.

I use H2 (##) on the first slide for the presentation title.

Every slides uses H3 (###) for the slide's title.

It looks like md2googleslides only uses H1 and H2 for slide titles and subtitles.

Would be good if instead it used the first header level it sees between --- separators. For example, if I have a H3 (###) as the first header after --- , treat that as the slide title, and H4 as the slide subtitle.

Alternatively, if it's too hard to detect per slide, allow at least setting it from the command line.