Add Open Help Option

Add `--open_help` option to open help on RTD for each command.

```

def get_help_option(self, ctx):
"""Returns the help option object."""
help_options = self.get_help_option_names(ctx)
if not help_options or not self.add_help_option:
return

def show_help(ctx, param, value):
if value and not ctx.resilient_parsing:
echo(ctx.get_help(), color=ctx.color)
ctx.exit()
return Option(help_options, is_flag=True,
is_eager=True, expose_value=False,
callback=show_help,
help='Show this message and exit.')
```

https://github.com/pallets/click/blob/master/click/core.py