Add `unsquashfs` utility to lib

[lovesegfault]

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Currently, if you want to replace a std::process::Command::new("unsquashfs") call with backhand, you need to replicate a large portion of the code in backhand-cli's unsquashfs bin.

It would be nice if the functionality of extracting a squashfs into a dir was provided by the library.

[wcampbell0x2a]

Surely. I already have backhand-cli for the next release, I could just extract the cli as-a- llibrary.

[lovesegfault]

I wanted to try my hand at this, what would you prefer:

1. Adding the utility to backhand's lib (will increase dependency closure)
2. Adding it to backhand under a helpers feature
3. Adding it in a new backhand-helpers crate

[wcampbell0x2a]

I haven't thought much about where the code would be, or what the API would be. But I expect the duplication to be the most between this package and backhand-cli. I fact, maybe just have the code be in that package, just as the lib.rs.

[wcampbell0x2a]

The API would mostly have the main of the bins, with the Clap argument structs as arguments? Just thinking out loud.