add new notes output to get command

[h0tw1r3]

Basically an alternate template to the changelog format that only works with a single "Entry" and doesn't output the changelog header or footer. A "full" changelog style entry is usually not desired when adding to annotated tags or a Github release message.

Ideally, as this extension is GH specific, I would like to see the release notes more release friendly (automatic contributors section).

Instead of this:

### Added
- Improve sections ordering https://github.com/chelnak/gh-changelog/pull/139 ([smortex](https://github.com/smortex))

Output this for release notes:

### Added
- #139: Improve sections ordering (@smortex)

Which ends up looking like:

[chelnak]

Nice! Check out my comment in your deps PR re an upcoming change.

What do you think about this?

Rather than --notes, I wonder if a flag like --output would be better?

For example, it leaves the implementation open for future types:

• --output notes
• --output json
• --output notes
• --output standard (default)

[h0tw1r3]

@chelnak all set I think

[chelnak]

Summing up my previous thoughts..

So yeah, I was going down the runtime route with my suggestions.

To be honest though, I'm perfectly happy with what you have in the PR.

My main concerns are always things like simplicity and whether the app follows the same patterns elsewhere!

[h0tw1r3]

From the comments, I don't think anything needs to change. Can this be merged?

[chelnak]

Let's do it 🙂