Adding ability to override variables in SASS version

Related issue  - https://github.com/erikflowers/weather-icons/issues/95

sass/weather-icons-core.scss

@@ -1,8 +1,8 @@
 // Base Variables
-$wi-path:                        "../font";
-$wi-version:                     "2.0";
-$wi-css-prefix:                  wi;
-$wi-font-family:                 weathericons;
+$wi-path:                        "../font" !default;
+$wi-version:                     "2.0" !default;
+$wi-css-prefix:                  wi !default;
+$wi-font-family:                 weathericons !default;
 
 @font-face {
     font-family: '#{$wi-font-family}';
@@ -50,4 +50,4 @@ $wi-font-family:                 weathericons;
 .#{$wi-css-prefix}-rotate-270 { @include wi-icon-rotate(270deg, 3); }
 
 .#{$wi-css-prefix}-flip-horizontal { @include wi-icon-flip(-1, 1, 0); }
-.#{$wi-css-prefix}-flip-vertical   { @include wi-icon-flip(1, -1, 2); }
+.#{$wi-css-prefix}-flip-vertical   { @include wi-icon-flip(1, -1, 2); }