Make taskbook available via Homebrew on macOS

**Is your feature request related to a problem? Please describe.**
Installing and upgrading the application via npm/yarn is inconvenient for people who are not web developers and don't have or know those tools. Homebrew is an established way to install command line utilities of all sorts on macOS and very widely used even by relatively non-technical people, making it a good fit for a general-purpose utility like taskbook.

**Describe the solution you'd like**
taskbook should be available via Homebrew on macOS and the readme should document installing via Homebrew.

Start with the README's current npm/yarn installation instructions and inspect how taskbook is packaged for command-line use. Determine the Homebrew distribution path, verify that taskbook can be installed and upgraded on macOS through it, and document the Homebrew installation in the README.

Written by the indexing model from the issue text.