Markdown processing

[matthiasbeyer]

The user should be able to use markdown markup in the changelog fragments and it should automatically be adapted for the final changelog file.

For example: They enter the following text for a fragment:

# Change API type: SessionToken

The SessionToken is now a `uuid` rather than a `u64`.

That would render (with the default template right now) to:

# CHANGELOG

## v0.1.0

# Change API type: SessionToken

The SessionToken is now a `uuid` rather than a `u64`.

But the Change API type: SessionToken heading should clearly be a ### heading.
This kind of processing should be done by cargo-changelog (IMO via a template helper).

[matthiasbeyer]

I guess it would be a better solution to provide generic text-modifying helpers for the template, rather than assuming markdown formatting.