Add an option to "include" function to increment headings

Add an option to the [include file](https://github.com/gajus/gitdown/tree/fab31aa75a95b0e9fa4b8d474ad42e853e8bf255#features-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`:

``` markdown
# README

## Section where LICENSE is included

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

Source `LICENSE.md`:

``` markdown
# LICENSE

Foo

## bar

Bar
```

Current output:

``` markdown
# README

## Section where LICENSE is included

# LICENSE

Foo

## bar

Bar
```

Desired output:

``` markdown
# README

## Section where LICENSE is included

### LICENSE

Foo

### bar

Bar
```