add basic support for configurable changelog entry types

[mikemorris]

Refs #6, but doesn't address any of the expected entry prefixes/RegEx types validation.

Configuration just uses a plain line-delimited text file for now in an effort to keep things simple and minimize dependencies, but would be open to considering HCL2/JSON/YAML config parsing instead if other maintainers think that would be a reasonable direction (looking for feedback specifically on if this would be worthwhile now).

Keeps the same default types for backwards compatibility, wires up custom type support into changelog-pr-body-check and changelog-entry through a slight tweak to the TypeValid function, and plumbs any custom types through to the interactive type selection in changelog-entry too.

TODO:

• TypeValues should probably be memoized rather than potentially loading a file on every call - not sure the most idiomatic way to do this.
• Would introducing HCL2/JSON/YAML config be preferable to the plain text file config?

/cc @bflad