Improve portability

[dag]

It might be useful to be able to use vim-fish with the tiny and small Vim builds; we should check for the relevant features with has() guards. Similarly, it could be useful to not require the latest version of Vim and at least support whatever old version is packaged for stable Linux distributions like Debian and Red Hat. This would mean fixing issues like #13.

It's debatable whether we should support running Vim in compatible mode, which would mean things like adding &cpo resets to every file.