Add new syntax for per-release optional metadata

[mergesort]

Currently the markdown spec is simple, using # prefixes for release information and - field: syntax for individual features.

# Version Number: [String] (Supports arbitrary number formats such as 1, 1.0, or 1.0.0)
## Release Title: [String]
### Semantic Version Change Type: [Major | Minor | Patch]

- title: [String] (Feature Title)
- description: [String] (Feature Description)
- symbol: [String] (SF Symbol Identifier)
- color: [String] (Hex Color Code)

As the desire to add fields to releases grows (such as a new release field for a header image (#1) or a new date field (#2)), there will need to be a way to define release-specific metadata. To accommodate new fields I propose adding a new #### prefix for optional metadata.

To specify individual metadata fields, the format would take inspiration from the feature syntax, creating a new format like this:

#### image: [String]
#### date: [String] (YYYY-MM-DD)

After that it should become be possible to add as many metadata fields to a release as necessary. These fields are optional, so if the parser does not parse a #### field, there will be a sensible default defined by the library.