Document use of HEROKU_APP in help text

[truthdoug]

Do you want to request a feature or report a bug?

feature

What is the current behavior?

N/A - Not a bug

What is the expected behavior?

When using --help for a subcommand or in error messages, I'd like to have instructions that instead of passing --app=app or -a app, that I can set the app in the HEROKU_APP environment variable.

For example, here's what updated help would look like for heroku logs --help:

display recent log output

USAGE
  $ heroku logs

OPTIONS
  -a, --app=app        (required) app to run command against (or set in HEROKU_APP)
  -d, --dyno=dyno      only show output from this dyno type (such as "web" or "worker")
  -n, --num=num        number of lines to display
  -r, --remote=remote  git remote of app to use
  -s, --source=source  only show output from this source (such as "app" or "heroku")
  -t, --tail           continually stream logs
  --force-colors       force use of colors (even on non-tty output)

DESCRIPTION
  disable colors with --no-color, HEROKU_LOGS_COLOR=0, or HEROKU_COLOR=0

EXAMPLES
  $ heroku logs --app=my-app
  $ heroku logs --dyno=web --app=my-app
  $ heroku logs --app=my-app --tail

  $ export HEROKU_APP=my-app
  $ heroku logs --num=50

Here's sample output when the --app option isn't passed:

$ heroku logs
 ›   Error: Missing required flag:
 ›     -a, --app APP  app to run command against, or set in the HEROKU_APP environment variable.
 ›   See more help with --help