Add next-version command

[wagoodman]

Adds the ability to speculate what the next release version might be from the set of changes discovered. This is expressed as two new features:

1. Adding a next-version command

# last github release: v0.1.0 
# state: one bug and one feature added since v0.1.0

$ chronicle next-version .
v0.2.0

2. Allow the create command to speculate the next version instead of report "unreleased"

# same release / repo state from (1)

$ chronicle > /tmp/changelog-1.txt
$ chronicle -n > /tmp/changelog-2.txt

# diff /tmp/changelog-1.txt /tmp/changelog-2.txt
3c3
< ## [(Unreleased)](https://github.com/wagoodman/chronicle-test/tree/4e685278ed9a268806c4d7fe534d2d58d6ca0d70) (2022-03-24)
---
> ## [v0.2.0](https://github.com/wagoodman/chronicle-test/tree/v0.2.0) (2022-03-24)
5c5
< [Full Changelog](https://github.com/wagoodman/chronicle-test/compare/v0.1.0...4e685278ed9a268806c4d7fe534d2d58d6ca0d70)
---
> [Full Changelog](https://github.com/wagoodman/chronicle-test/compare/v0.1.0...v0.2.0)