Version dependencies using dep

Rename `bump` command to `release`

Add --output optiont to write changes

Instead of using commits, use the `CHANGELOG.md` file to populate the
change log for this releas...

Merge duplicated sections on changelog when parsing

Fixes #17.

Fixes #15.

Create Changelog releases automatically

Refactor tests to use stretchr/testify assert library

Add basic bump subcommand

Useful when we can't automatically infer the URL.

Refactor parsing module

Rewrite the messy parser completely with tests.

Store them as structs instead and render only when needed

This makes them testable and isolates the spaghetti code.

This will make it easier (hopefully) to extract and reuse the same
struct/logic to bump a versio...

Outputs the markdown tree and populates the correct structs.