Clear or ignore cache

[pfrazee]

It'd be really helpful if I could either clear the cache via the CLI, or instruct electron-download to ignore cache during install via the API & CLI.

I have a workflow for a project, which involves republishing custom electron binaries without changing the name or version. I know, it's a little bit like this xkcd, but it saves me some headache if I can republish and just clear the cache automatically. (It lets me avoid making lots of commits that change a version number during development.)

A command which clears the cache, or a way to instruct the downloader to overwrite the cache, would be fine. Happy to make a PR based on your preference.