Mixed Setext and atx headers fail

[st-fankl-in]

I use setext style headers for the first 2 levels but bfdocs can't handle them :(
A file with mixed setext and atx style headers don't show up in the toc and the generated html isn't complete (fail.html from this example only contains Headline3 and Headline4)

Example:
manifest.json

{
    "title": "headline levels",
    "files": [
        "fail.md",
        "work.md",
        "kindofwork.md"
    ]
}

fail.md

Headline1
====
paragraph1

Headline2
----
paragraph2

### Headline3 ###
paragraph3

### Headline4 ###
paragraph4

work.md

# Headline1 #
paragraph1

## Headline2 ##
paragraph2

### Headline3 ###
paragraph3

### Headline4 ###
paragraph4

kindofwork.md

Headline1
====
paragraph1

Headline2
----
paragraph2