slt: expand 'info' command

Add command for extracting name, repo, and version.
slt: add 'semver' as wrapper for isaacs/semver CLI

Easier than messing with peerDependencies or other means of ensuring
the 'semver' command is in PATH.
slt-release: support relative versions

Allow 'major', 'minor', 'patch', etc. as version specifier for releases.

Closes #15