element().should().haveAttribute() for CSS attributes.

[dimazhiv]

Seems it would be useful if we were able to check CSS attribute through pattern like for simple element attributes: element().should().haveAttribute(nameAttribute, value)

For example:
element().should().haveCssAttribute(nameAttribute, expectedValue)
instead of using something like this:
assertThat(element().getCssValue("nameAttribute")).isEqualTo(expectedColor);

[vefimofff]

duplicates #77