Allow users to specify a filter function to remove endpoints from auto doc

[robertdzeigler]

I'm using httdomain on a tornado project with "auto tornado" to generate API documentation and it's great. A nice-to-have feature is the ability to specify a custom filter function to the autotornado directive to allow for greater dynamic control over which API routes are published vs. not. A few use cases include:

• Private/internal APIs endpoints that shouldn't be accessed by end-users
• "Unstable" APIs that are intended for eventual public consumption but should remain undocumented in official docs until the API has stabilized

I'd propose a "filter-endpoints" parameter to the auto tornado directive. It would take a tornado rule target (Rule or URLSpec) or a method/function target of such rule and return True to exclude the item (the item is filtered) and False to include it (the item is not filtered).

For instance, a filter function that excludes classes and methods which have "PRIVATE API" in the docstring:

def doctoring_private_filter(obj):
    return "PRIVATE API" in obj.__doc__

And would be setup as:

    .. autotornado:: yourwebapp:app
        :filter-endpoints: yourwebapp:docstring_private_filter

[stevepiercy]

@robertdzeigler this would be a nice feature. There's an open PR at #43 that could serve as a model. Would you like to make a PR?