feat: update version files when bumping the version

[crqrdotcom]

Fixes #1

• Introduces new flag --version-file (aka -f) on the bump command that accepts a list of files that should be updated when the version is bumped. The format is filename:key to make it easier to pattern match against.
• Before tagging the new version, the updated files are committed with the message bump: [previous version] -> [next version]. Every unstaged file is committed (similar to git add .), we might want to change this in the future.
• The package io/fs requires go 1.16 thus I'm deprecating support for go 1.15.

Examples:

git semver bump -f package.json:"version" pom.xml:<version>

[jsmvaldivia]

I've found some issues regarding perhaps readability