[Feature Request] Programmatic Usage

Hi everyone! `git-issue` is a great tool, and I've been using it for quite a while now. I decided to build a desktop app (I'll be releasing it soon) that makes it a bit easier to manage those issues.

I found it very hard to do certain things, like read *only the content*, *only the tags* or *only the author* from an issue.

I propose a command called, for example, `data`, that lets a user read only specific data or metadata from an issue. For example `git issue data --author` would simply print the author to the command line, `git issue data --tags` would simply print the tags of an issue to the command line, `git issue data --body` prints the body, etc. This would be incredibly useful as it would save me (and hopefully others too) all the work of writing regexes to parse the output of `git issue show ` into something programmatically meaningfull.