Fix '--help' behaviour

[laurensmiers]

Previously,
If a task did NOT have positional arg, passing '--help' to a task iso passing it to invoke directly worked as expected: --help would print the per-task help:

> invoke my_task --help # prints help of my_task which has no positional arguments

However, when passing '--help' to a task with (mandatory) positional arg, the string would just be passed to the task and executed.
This is not the expected behaviour of '--help', we would expect the per-task help to be printed.

With these commits,
we attempt to fix this issue by detecting when '--help/-h' is passed to a task and stop the parsing to print the per-task help.
Additionally, when a task is called without mandatory arg, the per-task help is now also printed.

As highlighted in #205, this will break any user-task having the --help OR -h cli options:

• --help
  As stated in previous discussions, users would rather expect --help to produce a help text
  than their task be called.
• -h
  This one is sadly a regression which will impact existing user scripts.
  However, IMO, this feels like a small price to pay for more unix-like tool behaviour of invoke.

As an example of what this PR behaves like, f.e. with this tasks.py:

from invoke import task

@task()
def no_arg(c):
    print(f"Invoked dummy!")

@task()
def one_arg(c, name):
    print(f"Invoked with param '{name}'!")

@task()
def two_arg(c, name, o_name):
    print(f"Invoked with param '{name}' and '{o_name}!")

Previous behaviour

• With '--help':

> inv no-arg --help  # prints help
> inv one-arg --help # prints "Invoked with param '--help'"
> inv two-arg --help # prints "'two-arg' did not receive required positional arguments: 'name', 'o_name'"

• Without '--help':

> inv no-arg   # prints 'Invoked dummy!'
> inv one-arg  # prints "'one-arg' did not receive required positional arguments: 'name'"
> inv two-arg  # prints "'two-arg' did not receive required positional arguments: 'name', 'o_name'"

New behaviour

• With '--help':

> inv no-arg --help  # prints help
> inv one-arg --help # prints help
> inv two-arg --help # prints help

• Without '--help':

> inv no-arg   # prints 'Invoked dummy!'
> inv one-arg  # prints "'one-arg' did not receive required positional arguments: 'name'" + help
> inv two-arg  # prints "'two-arg' did not receive required positional arguments: 'name', 'o_name'" + help

Feel free to propose different solutions, disagree with, comment on, discard, edit, use any of the proposed changes in this PR.