Title size decrease in nested subsections without a parent title

[traut]

Description

In the nested sub-sections the title gets shifted even when there is no title in the parent section. This creates gaps in title sizes, breaking formatting.

Steps to Reproduce

This template:

document "test_doc" {

  title = "AAA"

  section {
    title = "BBB"

    section {
      # No title

      section {
        title = "CCC"
      }
    }
  }
}

renders into

# AAA

## BBB

#### CCC

instead of correct

Expected Behavior

Expected gradual title descend:

# AAA

## BBB

### CCC

Actual Behavior

$ fabric --version
fabric version 0.4.2

[traut]

Using this issue to keep track of the title size problem introduced in #273 and blocked by #271