Generic font family check

New rule 'generic-font-family'

This rule checks, that each font/font-family declaration contains a 'generic' font family (one of the following: serif, sans-serif, cursive, monospace, fantasy).

Also, it is checks that 'generic' font family is specified as an identifier, not as a string, cause this prevents  correct rendering.

Specifying a 'generic' font family is needed to keep text rendering as near as possible to desired style on systems, where other specified fonts are not exists, some kind of a fallback.