refactor(grid): Update grid and docs based on feedback

* Added `*-xs-N` classes back.
  * `*-N` classes documented as shorthand for `*-xs-N`
* Ensure that wrapped columns have vertical gutter between them.
* Lots of updates to Grid documentation

Author: Ryan A. Johnson

source/components/grid/_explorer.less

@@ -6,11 +6,18 @@
   .hxCol,
   [class*="hxCol-"] {
     background-clip: content-box;
-    background-color: fade(@cyan-900, 25%);
+    background-color: fade(@green-700, 25%);
     font-size: 0.75rem;
   }
 }
 
+.drop-cap {
+  p::first-letter {
+    font-size: 2em;
+    font-weight: 700;
+  }
+}
+
 // Automatically Set Column Content based on breakpoint (lg-2, md-8, etc.)
 .demoColumns (@prefix, @n) when (@n > 0) and (@prefix = xs) {
   .demoCol.hxCol-@{n}::before {
@@ -62,3 +69,11 @@
   }
   .demoColumns(xl, @grid-column-count);
 }
+
+.styled-container {
+  background-color: @cyan-100;
+  border-radius: 0;
+  border: 1px solid @cyan-500;
+  height: 100%;
+  padding: 1em 0.5em;
+}

source/components/grid/_grid.less

@@ -1,6 +1,7 @@
 // ----- GRID MIXINS ----- //
 //e.g. .hxCol-1, .hxCol-md-8, .hxCol-lg-12
 .make-grid-columns (@prefix, @n) when (@n > 0) and (@prefix = xs) {
+  .hxCol-@{prefix}-@{n},
   .hxCol-@{n} {
     @width: (@n * @grid-column-unit);
     flex: 0 0 @width;
@@ -19,6 +20,7 @@
 
 //e.g. .hxOffset-1, .hxOffset-md-3, .hxOffset-lg-0
 .make-grid-offsets (@prefix, @n) when (@n >= 0) and (@prefix = xs) {
+  .hxOffset-@{prefix}-@{n},
   .hxOffset-@{n} {
     margin-left: (@n * @grid-column-unit) !important;
   }
@@ -33,6 +35,7 @@
 
 //e.g. .hxOrder-1, .hxOrder-md-4
 .make-grid-orders (@prefix, @n) when (@n > 0) and (@prefix = xs) {
+  .hxOrder-@{prefix}-@{n},
   .hxOrder-@{n} {
     order: @n;
   }
@@ -56,10 +59,11 @@
 //end:mixins
 
 /* ===== GRID ===== */
-@col-padding: (@grid-gutter-width * 0.5);
+@col-padding: (@grid-gutter-size * 0.5);
 @row-margin: -(@col-padding);
 
-.hxCol {
+.hxCol,
+[class*="hxCol-"] {
   display: flex;
   flex-direction: column;
   flex-grow: 1;
@@ -68,14 +72,13 @@
   border: 0;
   display: flex;
   flex-wrap: wrap;
-  margin-left: @row-margin;
-  margin-right: @row-margin;
+  margin: 0 @row-margin -(@grid-gutter-size);
   padding: 0;
 
   .hxCol,
   [class*="hxCol-"] {
     padding: 0 @col-padding;
-    margin: 0;
+    margin: 0 0 @grid-gutter-size;
   }
 
   &.no-gutters {