Add a fallback to alternate shards commands

[Willamin]

This allows additional functionality to be added to the shards command in a similar way extra commands are added to git. Basically, if the shards command isn't found in shards itself, it will run shards-#{command}, passing STDIO through. If there is no executable that matches the attempted command, the regular shards usage message is shown.

[ysbaddaden]

Any use cases?

[Willamin]

In short, extensibility.

Sometimes subcommands don't belong in the core shards project, but would be nice to have and run under the same shards umbrella. This allows the community to create and maintain subcommands without needing approval from the maintainers of shards itself.

For example, rust has implemented a similar scheme in cargo (https://github.com/rust-lang/cargo/wiki/Third-party-cargo-subcommands). Some good subcommand examples from this list include:

• audit (Audit lockfile for packages with security vulnerabilities)
• graph (Build GraphViz DOT files of dependency graphs)

Additionally, I would argue that any of these are valid use cases:

• Add/remove command (Add/remove command #192)
• Run command (Run command #157)
• Release command (New command: release #98)

[RX14]

I'm not all too sure of this. If your tool is separate from shards why not give it a separate command instead of stuffing it under the shards command? I imagine it's just confusing to users what's part of shards and what is not.

[ysbaddaden]

Well, git provides this and I don't think there are much confusion.

That being said shards-x and shards x are just one letter apart, and otherwise strictly equivalent —except that the latter may eat up some arguments...

[Willamin]

This PR's implementation should allow seamless passthrough of arguments.

[straight-shoota]

The benefit of having shards x vs shards-x is that it can be used to develop features individually and later adopt them into shards without changing the invocation.

[ysbaddaden]

I'll consider this for post v0.8.0 release.

[Sija]

We're already post v0.8.0, just a reminder ;)

[Sija]

@Willamin Are you willing to continue working on this PR?

[Willamin]

@Sija Yes, sorry for the delay, I'll implement the suggestions from @straight-shoota. 😄

[RX14]

I've never actually seen a git-foo executable not included in the standard git. Is this feature actually used there?

[jhass]

yeah. Two good examples are https://github.com/tummychow/git-absorb and http://hg-git.github.io/ https://github.com/cosmin/git-hg. If you want you can also count https://github.com/github/hub, which is supposed to be aliased to git and adds new subcommands.

[straight-shoota]

And a big collection of extra commands: https://github.com/tj/git-extras

[straight-shoota]

Thank you @Willamin