Add --version and --help flags to executables

[RKinsey]

Closes #74 , Closes #101
This patch adds flags for access to the usage message and the version tag/hash as requested in #74 and #57, respectively.

age and age-keygen with the --help or -h flag will print their usage message and exit, and age will also print usage when no flags or arguments are passed. Per #22, I didn't add age help.
age-keygen now has a usage message.

-v and (Removed, see below) --version work for both age and age-keygen, and will print the most recent git info at time of compilation. There are two build scripts (bash and Powershell) that grab the most recent tag and current short hash, and uses -ldflags to pass them to main.version and main.commit (named for compatibility with goreleaser defaults). Current format is

Version: [most recent tag]
Hash: [shorthash] 

and
Version: [most recent tag] when on a tagged commit or the --release flag is passed to the script.
I also updated the Homebrew formula for this, but it doesn't currently have an effect

[wgslr]

I'm glad to see you implemented my proposal. Just one note: I would be wary of using the -v flag for --version, as it is often used for --verbose and I could image age adding such flag in the future. I suggest either using -V or not providing a short flag at all, as the version invocation is done rarely and does not need to be particularly compact.

[RKinsey]

Good point, removed -v

[alerque]

@RKinsey I think you can also add Closes #101 to this issue description.

[RKinsey]

Got it

[Fastidious]

Would be great if this PR could be accepted. What say you, @FiloSottile? :-)

[brad2014]

Just a thought: There are two relevant version strings related to the age app: the release version of the code (currently "v1.0.0-beta2") and the version(s) of the supported file spec (currently "v1", but a later release might support multiple file specs).

Do you want to consider wording the age --help and the age --version output to distinguish between these two uses of the word version? Maybe:

% age --version 
Release version: 1.0.0-beta2
Age spec version: v1

[shepting]

I'd love to see this land! Any updates @RKinsey

[FiloSottile]

Most users will eventually use go install filippo.io/age/cmd/...@latest (which from Go 1.16 embeds the version) or Homebrew or a package manager, so I took a slightly simpler path in #173. Thank you!

@brad2014 that's a good point, but we don't plan to ever drop support for older versions of the spec, so for now I think it's fine not to expose it.

[ank-everstake]

I think it may be better to expose info for debug purposes.
User can open age-encrypted file on another machine with different age-binary.
So basically if we hide it we don't have any option to know what binary the user has.
You can compile by yourself, get pre-built binaries or get them through pacman AUR (in Arch).
Please consider to add the version back. Thanks.

CC @dd-everstake