Doc Build Warnings Should Cause Failures

[rmorshea]

https://travis-ci.com/rmorshea/idom/jobs/258370959

WARNING: Cannot resolve forward reference in type annotations of "idom.core.element.element": name 'VdomDict' is not defined
WARNING: Cannot resolve forward reference in type annotations of "idom.core.element.AbstractElement.mount": name 'AbstractLayout' is not defined
WARNING: Cannot resolve forward reference in type annotations of "idom.core.element.Element": name 'VdomDict' is not defined
WARNING: Cannot resolve forward reference in type annotations of "idom.core.element.Element.mount": name 'AbstractLayout' is not defined
WARNING: Cannot resolve forward reference in type annotations of "idom.server.base.AbstractRenderServer": name 'Element' is not defined
WARNING: Cannot resolve forward reference in type annotations of "idom.server.sanic.SanicRenderServer": name 'Element' is not defined
WARNING: Cannot resolve forward reference in type annotations of "idom.server.sanic.PerClientState": name 'Element' is not defined
WARNING: Cannot resolve forward reference in type annotations of "idom.server.sanic.SharedClientState": name 'Element' is not defined
WARNING: Cannot resolve forward reference in type annotations of "idom.widgets.common.EvalElement.mount": name 'AbstractLayout' is not defined
WARNING: Cannot resolve forward reference in type annotations of "idom.widgets.inputs.Input.mount": name 'AbstractLayout' is not defined
WARNING: Cannot treat a function defined as a local function: "idom.widgets.html.html"  (use @functools.wraps)
WARNING: Cannot resolve forward reference in type annotations of "idom.widgets.html.html": name 'EventHandlerFunction' is not defined

[rmorshea]

I have a sphinx extension that I've been using at work that I would like to open source to help fix these issues.

[rmorshea]

Completed in #409