Allow showing default values globally.

Feature request, as in #646, we already had a discussion how to easily show default values for options. As context_settings is already used to configure global settings anyway, the idea would be to allow the following:

@click.command(
    help='Command helptext',
    context_settings=dict(show_default=True))
@click.option("--test", default="foobar", help="Option helptext")
def cli(test):
    pass

Which should enable show_default for all options and give the following help text:

$ cli --help
Usage: cli [OPTIONS]

  Command helptext

Options:
  --test TEXT   Option helptext. Default: 'foobar'
  --help        Show this message and exit.

[lukeyeager]

Temporary workaround:

import functools
import click

# Use this instead of click.option
click_option = functools.partial(click.option, show_default=True)

@click.command()
@click_option('--name', default='Luke')
def say_hi(name):
    print(name)

if __name__ == '__main__':
    say_hi()

[ehles]

we have a lot of cli commands and a huge more params for them and going to show default, so we very appreciate for releasing this feature. anyway a year has passed since release 7.0, do you have any plans for releasing 7.1? Thanks.