Deprecate redundant metadata directives

Deprecates the `@author` and `@tagline` directives (#35), throwing specific error messages for each one that indicate what addon-info.json field to use instead. Also removes support for expanding `@plugin(tagline)` since the name "tagline" is no longer obvious and it's not that useful anyway. In the process, I discovered that we were accidentally suppressing errors for `@plugin(invalid)` and made those bubble up again.

Updated the README to document these fields and their behavior and removed documentation for the deprecated directives.