increase headings level for imported md documents

[vppuzakov]

Hi,

I have a problem with the headings of imported markdown documents. For example:

main.md:

# Main document

@import "child.md"

child.md:

# Child document

## Inner child header

This produces result.md:

# Main document

# Child document

## Inner child header

Expected result:

# Main document

## Child document

### Inner child header

[tmori3y2]

I sometimes use @import directive to separate large documents into sections.

main.md

# document title {ignore=true}

# Overview {ignore=true}

[TOC]

@import "./section_1.md"
@import "./section_2.md"
@import "./section_3.md"

section_1.md

# Section  1

## Section  1.1

some contents here...

This PR does not fit into existing use cases and I cannot accept it as a user of MPE.

You can solve this problem by adding additional attributes to @import directive. Of course, this feature is disabled by default.

It is recommended that you create an issue to discuss what options should be added.