Add -k/--known to list known Go versions

[ellotheth]

I found myself visiting the Go release notes page to see which patch releases were available for specific releases. This PR makes that list available with gimme -k.

I used https://golang.org/dl as the base list URL instead of GIMME_DOWNLOAD_BASE because the latter list doesn't include the (current) 1.9 beta. The golang.org list doesn't include old betas or release candidates, so maybe a combination of the two would be a better approach?

[philpennock]

I like it. One nit and one open question, neither of which blocks merge IMO but the question is around behavior where we wouldn't want to bumble around post-merge so if we change then defer merge.

nit: | grep . to get rid of the blank line in the output (or use arrays in the first place), so that we only output lines with version numbers on -- this is a nicer API to offer for scripting.

question: what should happen about locally-installed-but-not-available versions? This PR declares known to be "what could be installed if not already installed locally", but old betas from previous versions might be available for activation. So getting a list of files from ${GIMME_ENV_PREFIX} and matching via regexp against those, adding to the list, and adding -u to the sort to uniq would augment the list of "known".

So as it stands, this is "installable", not "known". Options are (1) we're totally happy with that, merge as is; (2) we're happy with behavior, rename the option and merge; (3) augment the behavior to merge installed versions first.

@meatballhat and @ellotheth ? Thoughts?

And @ellotheth if we want changes let us know if you're happy making them yourself, or if you'd rather they be done separately. I like the changed behavior and would be happy to pick up, but don't want to barge in unwanted.

[ellotheth]

@philpennock Thanks for the review and feedback!

nit: | grep . to get rid of the blank line in the output

👍

what should happen about locally-installed-but-not-available versions?

You're right, known should mean more than versions available on the official site. Can I cheat and add the existing _list_versions output to _list_known?

[philpennock]

Thanks!

[ellotheth]

🎉 Most welcome!