[feature request] Support .backpack.d

[pallxk]

I'm using backpack for multiple projects and hosts, and want to load different snippets for them.

It'd be great if I could save those snippets into separate files in ~/.backpack.d/*, or more specifically ~/.backpack.d/*.backpack, and do check and custom logic within.

I can file a PR for this, but not sure if it's the thing we want for the project, so put an issue here for discussion first.