Expand semver logic to include NPM semantic versioning rules.

[alilleybrinker]

Right now we only support Cargo's semantic versioning rules, but it would be even better if semver-explain could explain the rules of other systems as well. To start with, we could work on supporting the NPM rules.

This will involve adding a flag (options below) to specify which flavor is being used.

Options:

• -k/--kind
• -f/--flavor
• -s/--style

Some of these flags overlap with well-known patterns for flag naming which may be undesirable. It's worth reviewing the CLI Guidelines site for prior art.