Support updating version files on version bump

[crqrdotcom]

Goal:

• Support updating the version files when bumping the version (i.e. package.json, Cargo.toml, etc.)
• Be flexible enough to not be coupled to certain version files and, instead, allow the user to specify which files they want to update

Why:

• Increase automation
• Make it easier to bump a project's version

How:

• Add new --version-files option to git semver bump command so users can specify which files they want to update
• On each file specified by --version-files, replace the old version with the new one using (through a naive and simple match against the old version)

Implementation Caveats:

• The version files need to have a correct version set to the latest released version on the first run
• The version in the version files must have a higher priority meaning that they'll need to appear before any other string that matches exactly the latest released version

Additional:

• Document the new option and implementation caveats