Add CTA logo at top of spec next to W3C logo

[mavgit]

ReSpec has a feature called logos to add alternate or additional logos to the top of the spec. I tried using this to add the CTA logo to the CG 2018 API spec, since the CTA publication of the 2017 API spec has both logos. However, there seems to be a ReSpec bug that makes the formatting come out wrong. @tidoust filed a bug on this: w3c/tr-design#170

@tidoust also gave me a workaround to add a style sheet, which I will apply to the spec, since it is purely editorial..