Customize path: Allow default repo, default branch

[Erudition]

It would be nice if instead of this:

https://username.github.io/preview-repo/username/preview-repo/branch/master/

I could have this:

https://username.github.io/preview-repo/

which is much more reasonable for a base URL for a webapp.

• the default branch can be selected, e.g. master/main/latest/release or whatever, that bypasses the `/branch/master/ extensions
• in the case of single repo usage, or with a default repo selected, the redundant /username/preview-repo can be dropped too.
• some might also use it for multiple repos, but they're all authored by themselves, so might as well drop the repo author bit in that case.

In all other cases, the full path works fine.

It would also be nice to be able to customize e.g. /pr/number/ to /with-pr/number or even just /number, and the same for branches.

Thanks for this super useful project!

[EndBug]

That's a great idea!
We could add a preview_relative_path input so that users can override the default path: this way we don't risk introducing any breaking changes, we don't have to deal with all the possible scenarios, and we're sure that it's 100% customizable ✅