Consecutive headers need a newline between them

[devrelm]

When creating a Release on GitHub with two headers in the description like so:

## Released

### Changed

the /integrations/slack integration created a slack post like this:

My guess is that the markdown is being converted to an <h2> and <h3> with no space between them, and that html-to-mrkdwn is not going out of its way to add a newline back.

Because the <h1> through <h6> tags default to display: block, they naturally stack when viewed in a browser. As such, the expected behavior should be that headers – once converted – always begin and end with a newline.