Consequence: you can no longer install when something is already installed, you must upgrade it. This doesn't apply if the formula in question was unlinked. You can still --force installs though.

Rationale: the old way of installing over the top would leave symlinks to multiple versions in /usr/local if the old version had a file the newer version didn't. The new upgrade command handles everything properly.

Be more useful by being more concise. I referenced dozens of other mature commands’ usage to figure out what to do here.

Also separated out the help into its own command for consistency.