This fixes a bug where lines starting with a space would get parsed as
"level 0" headings - it would not find a "#" and therefore never
increase the level counter (starting at zero), which then would cause
the check for "space after #" pass (again, there is no "#").
Eventually we would get funny results like this:
<h0>[n-1 spaces]oops!</h0>
Also ASSERT(level > 0) in the Heading constructor.
Previously, all Markdown blocks had a virtual parse method which has
been swapped out for a static parse method returning an OwnPtr of
that block's type.
The Text class also now has a static parse method that will return an
Optional<Text>.
