Whitelist CSS modules from purge

PurgeCSS removes unused CSS from the output. This PR only uses PurgeCSS on the base utility classes to avoid unexpected issues where JS or content-based classes get removed.

resources/_gen/assets/css/output/css/app.css_d11fe7b62c27961c87ecd0f2490357b9.json

@@ -1 +1 @@
-{"Target":"output/css/app.min.00b63e6932f074f9d50f0b897874add78a83b7599089f59d6a5400a75276af9c.css","MediaType":"text/css","Data":{"Integrity":"sha256-ALY+aTLwdPnVDwuJeHSt14qDt1mQifWdalQAp1J2r5w="}}
+{"Target":"output/css/app.min.3d04cd530b6fbca129e24fb79af6692fa64cad11185196c7a800e58b4b3e2f97.css","MediaType":"text/css","Data":{"Integrity":"sha256-PQTNUwtvvKEp4k+3mvZpL6ZMrREYUZbHqADli0s+L5c="}}

themes/gohugoioTheme/assets/css/main.css

@@ -1,10 +1,10 @@
 /*Base Styles*/
 @import '_tachyons';
 
+/* purgecss start ignore */
 @import '_anchorforid';
 @import '_animation';
 @import '_documentation-styles';
-
 @import '_algolia';
 @import '_carousel';
 @import '_code';
@@ -16,7 +16,6 @@
 @import '_definition-lists';
 @import '_fluid-type';
 @import '_font-family';
-/*@import '_hljs';*/
 @import '_hugo-internal-template-styling';
 @import '_no-js';
 @import '_social-icons';
@@ -36,3 +35,4 @@
 .mw-90 {
   max-width:90%;
 }
+/* purgecss end ignore */