@click.argument don't support help parameter

[Ralph-Wang]

@click.command()
@click.argument("arg", help="help message")
def hello(arg):
    pass

hello()

The error message

Traceback (most recent call last):
  File "x.py", line 7, in <module>
    @click.argument("arg", help="help message")
  File "/Users/ralph-wang/.pyenv/py3/lib/python3.6/site-packages/click/decorators.py", line 151, in decorator
    _param_memo(f, ArgumentClass(param_decls, **attrs))
  File "/Users/ralph-wang/.pyenv/py3/lib/python3.6/site-packages/click/core.py", line 1699, in __init__
    Parameter.__init__(self, param_decls, required=required, **attrs)
TypeError: __init__() got an unexpected keyword argument 'help'

It seems that we don't support help message in argument. Is that designed for?

[Ralph-Wang]

I post a PR to support help message for argument, as I thinks it's an important feature that we have ability to print some detail message for arguments

[jcrotts]

Closed, see #1051 for discussion.