[Feature request] On 404, fall back on a default branch or the repo root url

[hlissner]

When browse-at-remote encounters a file whose url cannot be resolved (e.g. is untracked, or on an untracked branch, etc), an error occurs and that's that. I'd like to suggest one of the following as alternatives:

1. It falls back to a default branch. Either master, or from a variable like browse-at-remote-default-branch.
2. Have browse-at-remote/get-url or browse-at-remote/file-url accept an optional branch argument, then we can write it ourselves.
3. Or it falls back to the root of the repository.

The rationale is that when I use browse-at-remote, I have a good idea of where I want to end up. If it doesn't find the exact url, I find it preferrable that it take me somewhere close to the target, rather than stop me dead in my tracks with an error.

I'd be happy to do a PR for this, if you'll let me know which approach (or another) you'd prefer.

Let me know! And thanks for this plugin by the way!