feat: option for cloudflared access login to only output jwt (#1249)

[rjgpacheco]

Adds a -no-pretty flag to cloudflared access login:

$ cloudflared access login -no-pretty $url
<TOKEN>

This removes the need to parse the token from the output message, making it easier for external applications.

This is a pretty small change change that makes it much simpler to use from, eg Python script.

With the current CLI, the caller must manually parse the output, eg:

from subprocess import check_output

def get_auth_token(url) -> str:
    cmd = ["cloudflared", "access", "login", url]
    return check_output(cmd).decode("utf-8").strip().split()[-1]

Python's the simplest case here, but it's a step that doesn't really need to happen.

Also: the hardest part is coming up with a name for the flag, so I didn't thing too much about it, open to suggestions :)

[janrueth]

--only-token might be an alternative. But I like this feature, makes using cloudflared in bash scripts a lot simpler