More compliant Keep a Changelog parser

[colincasey]

This is still a work-in-progress but it's my first pass at addressing #153 with an alternative parser that is more compliant with the documented format.

It takes in the content of a changelog that should be in a flattened structure like the following:

# Changelog            → (Changelog)
                       → -
## Unreleased          → (ReleaseEntry::Unreleased)
                       → (ReleaseContents)
## [x.y.z] yyyy-mm-dd  → (ReleaseEntry::Versioned)
                       → (ReleaseContents)
### Changed            → (ChangeGroup)
                       → (List)
- foo                  → (List Item)
- bar                  → (List Item)
                       → -
### Removed            → (ChangeGroup)
                       → (List)
- baz                  → (List Item)

And convert that into the following structure:

Changelog {
  unreleased: None,
  releases: [
    ReleaseEntry {
      version: x.y.z,
      date: yyyy-mm-dd,
      tag: None,
      contents: ReleaseContents {
        "Changed": ["foo", "bar"],
        "Removed": ["baz"]
      }
    }
  ]
}

[joshwlewis]

This is very cool. I'd love to use it on github.com/heroku/buildpacks-nodejs when generating changelog entries for automatic inventory updates!