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/custom/_fonts.scss

@@ -0,0 +1,9 @@
+// Here you can easily change font faces which are used in your site.
+// To give it a try, uncomment some of the lines below rebuild your blog, and see how it works. your sites's.
+// If you love to use Web Fonts, you also need to add some lines to source/_includes/custom/head.html
+
+//$sans: "Optima", sans-serif;
+//$serif: "Baskerville", serif;
+//$mono: "Courier", monospace;
+//$heading-font-family: "Verdana", sans-serif;
+//$header-title-font-family: "Futura", sans-serif;