Add a blacklist option for member attributes

[Erik-White]

I found that it would be very useful to have a blacklist option for :members:, instead of the current whitelist option. I've added an extra command :members-exclude: that renders identically to :members:, but excludes any listed attributes

Example usage:

::: mocklib.ExampleClass
    :docstring:
    :members-exclude: example_property

[Erik-White]

Updated to fix typo in commit message

[Erik-White]

@tomchristie any thoughts?

[tomchristie]

Thanks but I'd prefer to stick with things how they are.