Add an option to "include" function to increment headings

[gajus]

Add an option to the include file function that would increment heading size of the document being included to the size of the host section heading.

Example:

Source README.md:

# README

## Section where LICENSE is included

{"gitdown": "include", "file": "./LICENSE.md","adjustHeading":"contain"}

Source LICENSE.md:

# LICENSE

Foo

## bar

Bar

Current output:

# README

## Section where LICENSE is included

# LICENSE

Foo

## bar

Bar

Desired output:

# README

## Section where LICENSE is included

### LICENSE

Foo

### bar

Bar