tfinstall cmd

[kmoe]

closes #15

First pass at a tfinstall binary. Current usage:

Usage: tfinstall [--dir=DIR] VERSION

  Downloads, verifies, and installs a terraform binary of version
  VERSION from releases.hashicorp.com. VERSION must be a valid version under
  semantic versioning, or "latest".

  If a binary is successfully installed, its path will be printed to stdout.

  Unless --dir is given, the default system temporary directory will be used.

Options:
  --dir          Directory into which to install the terraform binary. The
                 directory must exist.

Examples:
  tfinstall 0.12.28
  tfinstall latest
  tfinstall 0.13.0-beta3
  tfinstall --dir=/home/kmoe/bin 0.12.28

@paultyng @radeksimko looking for feedback on the UX here. This is intended as a simple utility for use in CI build steps. When we're happy with it I can add config to build and release binaries as part of the release process.

Implemented using a somewhat bastardised version of mitchellh/cli, because I wanted cli.Ui but there's currently no way to have flags without subcommands (mitchellh/cli#24).

[paultyng]

The UX looks good to me, the only thing I would probably prefer would that no args/flags at all (ie. just tfinstall) is equivalent to tfinstall latest.

[radeksimko]

the only thing I would probably prefer would that no args/flags at all (ie. just tfinstall) is equivalent to tfinstall latest.

I would disagree here. There's a number of Unix-based tools which bring up help page when executed without arguments, I think that is a safe default.

The use cases we are IMO mostly targetting with this is CI, where you set it up once and forget about it - so you'd usually take care to study supported flags anyway before making it run on every PR and explicitness is what you'd prefer there too.

Lastly I think that installing latest is probably a bad idea in most cases where there is an existing config, so we shouldn't be encouraging it.

[kmoe]

@paultyng this is ready to merge now in preparation for future work. I'll add the goreleaser config (#71) in a separate PR.

[paultyng]

Just rebased and go mod tidy on this in my update.