Add some built-in faces for inheritance

[Yuan Fu]

I think it would be helpful to define some common UI concepts as faces, so 
package authors can inherit from them instead of hard-coding their face. We 
already have faces like shadow, bold, error, warning, info, button, link. We 
can add a few more:

- foreground-highlight: highlight text using a different foreground color, used 
for matched results, etc.
- background-highlight: highlighted text using a different background color, 
used for selected candidate in company/ivy/helm, etc.
- block: text with a different background to separate from main text, used for 
code blocks, input fields, pop ups, etc
…

It would be helpful if someone with UI-design background could provide some 
ideas.

Yuan