Fix vanishing MMD-starting image caption

This MMD code should have a `<figcaption>` tag when parsed, but doesn’t:

    ![*test* me](http://example.com/)

This one does, though (notice the leading whitespace):

    ![ *test* me](http://example.com/)

In short, if the first caption character is part of the MMD syntax, it vanishes completely; this should fix it while keeping #58 in place.