The `NO_COLOR` environment variable should disable all ANSI color
output (https:://no-color.org).

This commit updates `fmt::Subscriber` to check the `NO_COLOR`
environment variable before enabling ANSI color output.  As described in
the spec, any non-empty value set for `NO_COLOR` will prevent `is_ansi`
from being set to `true`.

fixes tokio-rs#2388

Re-reading https://no-color.org showed that they do allow overriding of
the setting with command-line flags.  Similarly, I've removed the
changes to `fmt::subscriber::with_ansi` and `fmt::subscriber::set_ansi`;
they will once again set `is_ansi` regardless of `NO_COLOR`.

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

…' into subscriber-no_color