Makes font-family names customizable.

This chagne instroduces following variables
that allows authors to customize a font family for each class.
- $serif
- $sans
- $heading-font-family
- $header-title-font-family

This change also adds anotehr custom scss file named "_fonts.scss".
We can use it to override these newly introduced variables.

.themes/classic/sass/screen.scss

@@ -3,6 +3,7 @@
 @include reset-html5;
 
 @import "custom/colors";
+@import "custom/fonts";
 @import "custom/layout";
 @import "base";
 @import "partials";