Add FlakeUpdate command

[OliverEvans96]

Closes #47.

I basically just copied and pasted the FlakeMetadata implementation. I imagine at some point the flake subcommands might be refactored into their own namespace, but it seems appropriate to just submit the simplest PR first.

As a side-node: As I was testing, I needed to have the PARSER_UTIL_BIN env var set to build successfully. Is this binary dependency new since the latest released version? I was able to get it working by snooping around your nix files, but it seems like it might be somewhat of a barrier to adoption. Maybe it could at least be discussed in the readme.

Let me know if you'd like any changes to the PR.

Thanks,
Oliver

[ysndr]

Hej Oliver, thanks for the contribution.

I agree the commands might need to organized a bit differently as we're adding on more, but for now its simple enough to just collect them in a single namespace, since they are rather mostly boilerplate as you probably noticed.

The PARSER_UTIL_BIN is a regression from our recent move to directly use nix' uri parser rather than reimplementing it. The adoption hurdles that come with it are unfortunate, and documenting them is the least we can do.

[mkenigs]

The PARSER_UTIL_BIN is a regression from our recent move to directly use nix' uri parser rather than reimplementing it. The adoption hurdles that come with it are unfortunate, and documenting them is the least we can do.

Opened #51