Allow configuring a name for the archived binary

[magni-]

This will default to the existing behavior: using the name of the currently running executable.

I don't write much Go, so hopefully I didn't do things too terribly 😬

Fixes #42

[magni-]

(Noticed the build failed but I'm at the end of my day, will take a closer look tomorrow 🙇🏼)

[magni-]

FWIW, this is merged in our fork (rainforestapp/go-github-selfupdate) and used by our CLI (see PR implementing it here: https://github.com/rainforestapp/rainforest-cli/pull/413/files), so I can confirm this works in the real world 😅

[wtrocki]

Yes. This repo seems to be no longer maintained. I will move to use yours @magni-