Force binary existance check

[sstadick]

This PR allows for explicit checking for binary existence when using the associate and try_associate methods on the DecompressionMatcherBuilder. I have tried to preserve the current resolve_binary api being a no-op on non-windows as well.

Currently, a use tries to {try_}associate a program on a non-windows OS, resolve_binary no-ops and never checks the path to see if that binary actually exists or not.

This change forces a path check for existence even on non-windows OS so that a user can try to associate several different binaries with a glob and only the binaries found on the system will be associated.

Concrete example: https://github.com/sstadick/hck/blob/f5aca37b0a06523074fe229fa77bb863e4f8574c/src/lib/core.rs#L365

I want to associate pigz with *.gz only if pigz is installed on the users system, otherwise it can fail and gzip can be used. Currently pigz is not actually tested for existence, it's added as a matcher and then fails to find it when called resulting in no decompression happening.