Make waypoint config set command pipeable

[dominikbraun]

Fixes #561.

This change allows the user to pass arguments to waypoint config set using pipes:

$ cat .env | waypoint config set

---

The linked issue also proposes a -raw flag for config get, but this flag apparently already exists:

waypoint/internal/cli/config_get.go

Lines 137 to 141 in b00f8ab

	f.BoolVar(&flag.BoolVar{
	Name: "raw",
	Target: &c.raw,
	Usage: "Output the value for the named variable only (disables prefix matching)",
	})

However, waypoint config get -raw doesn't print anything to my console. Maybe this is a separate issue.

[mitchellh]

This looks pretty good! Thank you! I requested some slight changes.

[mitchellh]

Nitpick: I would change this a bit to set configArgs := c.args earlier up and then just append to that here. That way we don’t need the somewhat awkward else fall through.

[dominikbraun]

Absolutely, that's a more elegant solution.

[dominikbraun]

So I just noticed that I'm now directly appending the config variables to c.args without creating a new slice configArgs at all - but you actually suggested to initialize configArgs with c.Args and still append the variables to configArgs.

Which way would you prefer?

[mitchellh]

I think its fine as-is since we verify len(c.args) == 0 to guard entry into that path anyways. Thanks!