Add tag command with Semver release support

[joseluisq]

Add tag command to create Git tags following The Semantic Versioning 2.0.0 and supporting the following options:

Proposal

# show all tags available
~> tag

# show all tag command flags
~> tag -h
-n --new-version [version]  tag a new version
-x --major                  tag auto-incrementing a major version number
-y --minor                  tag auto-incrementing a minor version number
-y --patch                  tag auto-incrementing a patch version number

Examples

~> tag --new-version v1.0.0
# v1.0.0
~> tag --major
# v2.0.0
~> tag --minor
# v2.1.0
~> tag --patch
# v2.1.1