Begin adding sources for color guidance.

Sources were added for the following colors. Brands that are bolded also had their color values changed in accordance to their guidelines.
 * Facebook
 * **Google+**: *There were two entries named `google`. One of them matched the color guidance for Google+, so I renamed it to `googleplus`.*
 * **LinkedIn**
 * **Pinterest**
 * Quora
 * **Tumblr**
 * Twitter
 * **YouTube**