Support for PR suggestion commit message too

[hjkatz]

Currently a suggestion can be left as such:

```suggestion
return fmt.Errorf("It done broke: %w", err)

This is great as it allows the PR author to quickly apply commits with the suggested changes. However, the commit message for these suggestions are standard and generic like: "Update <path/to/file>". This puts burden on the button clicker to create a nice commit message (such as from: https://chris.beams.io/posts/git-commit/). This burden is very small, however, any resistance could possibly limit an author from clicking the button due to the minimal cognitive overhead of coming up with a nice commit message that describes the suggested change.

Further, the author of the PR is not the commenter of the suggested change. So while the commenter may have had an idea for the suggestion and commit message in mind, the author of the PR must read the mind of the commenter to create a nice commit message.

May I suggest that the burden of the commit message be placed on the commenter of the suggestion instead? Perhaps with this syntax:

```suggestion "Use go 1.13 errors"
return fmt.Errorf("It done broke: %w", err)

Now when the author clicks the button to accept suggestion the pre-populated commit message should be "Use go 1.13 errors" which would be a nice nit commit message.

Pretty please and thank you :)