Add better system fonts (#205)

Add a system font stack to support different OSes for a more comfortable & pleasing look. This exact stack is used by https://github.com/ as well

Author: Yash Ghelani

_sass/minima.scss

@@ -2,7 +2,7 @@
 
 // Define defaults for each variable.
 
-$base-font-family: "Helvetica Neue", Helvetica, Arial, sans-serif !default;
+$base-font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol" !default;
 $base-font-size:   16px !default;
 $base-font-weight: 400 !default;
 $small-font-size:  $base-font-size * 0.875 !default;