New testing assertions for HTML attributes.

Allows the testing for attributes:

    response.Body["#someElement"]
        .ShouldExistOnce()
        .And.ShouldContainAttribute("href", "http://whatev.er");

Which is easier to read than this:

    response.Body["#someElement[href='http://whatev.er']"].ShouldExistOnce();

And easier to type than this (using FluentAssertions):

    response.Body["#someElement"]
        .ShouldExistOnce()
        .And.Attributes["href"].Should().Be("http://whatev.er");

It also allows testing for attribute existence and tests on multiple nodes:

    response.Body["a"]
       .ShouldContainAttribute("href");