Enable users to use "preview-url" in the "github-comment"

[peteroid]

Hi there, it would be great if we can customize the comments with the preview url. I think there are two ways:

1. Allow template string in "github-comment"
   • So users can it as "I have deployed {{previewName}} at {{previewUrl}}".
2. Create the githubComment after we have set the output of "preview-url"
   • Currently, the githubComment was created at the very beginning of the action.
   • The function of building comment is called after the "preview-url" has been set and it should be possible to use that output when we get the githubComment in building function.

Follow up of #52