Start autocomplete generation for fish

[mrmeszaros]

Added complete command that generates basic auto-completions for fish.
Other shells are not supported yet.

Fixes #753

[zimbatm]

I think I would rather convert direnv to use urfave/cli or another CLI library that already supports completion out of the box. At the time where direnv was written, those didn't exist.

[zimbatm]

Alternatively, ship the completion script as part of the install. Or does it require to interact with the binary?

[mrmeszaros]

@zimbatm Great idea, I love CLI libraries (I use click for python <3).
They make it easy to keep help / man page uniform and consistent with the app.

I am rather new to Go, but I will look into it.
Here they recommend Cobra for CLI building (and Viper for config management).
I will check out urfave/cli and try to compare them.

Cobra seems to support generating completion for bash, zsh, fish and PowerShell. No elvish yet, but its pre-1.0 so it may change in the future. No tcsh either, but I am unfamiliar with that shell's popularity.

As for a static autocomplete script.
I thought that having the help text in two different locations - the command file and the completion script - would make it easier to de-synchronize. I saw a few projects use that kind of thinking. Might be overthinking though (eg. git has them separate) - the interface should not change that often to matter.

[mrmeszaros]

Ok, so another search resulted in a "curated" list of CLI libraries - so even more choices to make I guess.

Now, do You have any measures / priorities to optimize for? Like future support plausibility / performance / readability?

[zimbatm]

I had a look at the list and https://github.com/mkideal/cli looks pretty good. There is so much choice, I am sure other options would probably be fine as well.