Check for binaries on launch

[popey]

When grummage launches, it should check for the grype binary in the $PATH and the current directory.

If the grype binary is not found, we should display an error and stop execution as a minimum effort.

In the best-case scenario, let's offer to download grype using the official installation instructions (curl piped to bash) to the current directory.

In a future release, we could have an environment variable that points to the grype binary rather than having to hunt for it. I am open to discussion about this one.

[shaunsetlock]

Hi @popey !

I heard you mention this small project on Linux Matters and thought it would be something I could help with. 🙌

Be on the lookout for a PR from me.

Shaun

P.S. - I'm not a professional Python developer, so very open to feedback!